Truly random number generator: Turing computable? I am seeking a definitive answer to whether or not generation of "truly random" numbers
is Turing computable.  I don't know how to phrase this precisely.
This StackExchange question on "efficient algorithms for random number generation"
comes close to answering my question.
Charles Stewart says in his answer, "it [Martin-Löf randomness] cannot be generated by a machine." Ross Snider says, "any deterministic process (such as Turing/Register Machines) can not produce 'philosophical' or 'true' random numbers." Is there a clear and accepted notion of what constitutes a truly random number generator?
And if so, is it known that it cannot be computed by a Turing Machine?

Perhaps pointing me to the relevant literature would suffice.  Thanks for any help you can provide!

Edit. Thanks to Ian and Aaron for the knowledgeable answers!  I am relatively unschooled in this area, and I am grateful for the assistance. If I may extend the question a bit in this addendum: Is it the case that a TM with access to a pure source of randomness (an oracle?), can compute a function that a classical TM cannot?
I am joining the discussion fairly late, but I will try to address several questions that were asked earlier.

First, as observed by Aaron Sterling, it is important to first decide what we mean by "truly random" numbers, and especially if we are looking at things from a computational complexity or computability perspective.

Let me argue however that in complexity theory, people are mainly interested in pseudo-randomness, and pseudo-random generators, i.e. functions from strings to strings such that the distribution of the output sequences cannot be told apart from the uniform distribution by some efficient process (where several meanings of efficient can be considered, e.g. polytime computable, polynomial-size circuits etc). It is a beautiful and very active research area, but I think most people would agree that the objects it studies are not truly random, it is enough that they just look random (hence the term "pseudo"). 

In computability theory, a concensus has emerged to what should be a good notion of "true randomness", and it is indeed the notion of Martin-Löf randomness which prevailed (other ones have been proposed and are interesting to study but do not bare all the nice properties Martin-Löf randomness has). To simplify matters, we will consider randomness for infinite binary sequences (other objects such as functions from strings to strings can easily be encoded by such sequence). 

An infinite binary sequence $\alpha$ is Martin-Löf random if no computable process  (even if we allow this process to be computable in triple exponential time or higher) can detect a randomness flaw. 

(1) What do we mean by "randomness flaw"? That part is easy: it is a set of measure 0, i.e. a property that almost all sequences do not have (here we talk about Lebesgue measure i.e. the measure where each bit has a $1/2$ probability to be $0$ independently of all the other bits). An example of such a flaw is "having asymptotically 1/3 of zeroes and 2/3 of ones", which violates the law of large numbers. Another example is "for every n, the first 2n bits of $\alpha$ are perfectly distributed (as many zeroes as ones)". In this case the law of large numbers is satified, but not the central limit theorem. Etc etc.
(2) How can a computable process test that a sequence does not belong to a particular set of measure 0? In other words, what sets of measure 0 can be computably described? This is precisely what Martin-Löf tests are about. A Martin-Löf test is a computable procedure which, given an input k, computably (i.e., via a Turing machine with input $k$) generates a sequence of strings $w_{k,0}$, $w_{k,1}$, ... such that the set $U_k$ of infinite sequences starting by one of those $w_{k,i}$ has measure at most $2^{-k}$ (if you like topology, notice that this is an open set in the product topology for the set of infinite binary sequences). Then the set $G=\bigcap_k U_k$ has measure $0$ and is referred to as Martin-Löf nullset. We can now define Martin-Löf randomness by saying that an infinite binary sequence $\alpha$ is Martin-Löf random if it does not belong to any Martin-Löf nullset.  

This definition might seem technical but it is widely accepted as being the right one for several reasons:


it is effective enough, i.e. its definition involves computable processes
it is strong enough: any "almost sure" property you may find in a probability theory textbook (law of large numbers, law of iterated logarithm, etc) can be tested by a Martin-Löf test (although this is sometimes hard to prove)
it has been independently proposed by several people using different definitions (in particular the Levin-Chaitin definition using Kolmogorov complexity); and the fact that they all lead to the same concept is a hint that it should be the right notion (a little bit like the notion of computable function, which can be defined via Turing machines, recursive functions, lambda-calculus, etc.) 
the mathematical theory behind it is very nice! see the three excellent books An Introduction to Kolmogorov Complexity and Its Applications (Li and Vitanyi), Algorithmic randomness and complexity (Downey and Hirschfeldt) Computability and Randomness (Nies).


What does a Martin-Löf random sequence look like? Well, take a perfectly balanced coin and start flipping it. At each flip, write a 0 for heads and a 1 for tails. Continue until the end of time. That's what a Martin-Löf sequence looks like :-) 

Now back to the initial question: is there a computable way to generate a Martin-Löf random sequence? Intuitively the answer should be NO, because if we can use a computable process to generate a sequence $\alpha$, then we can certainly use a computable process to describe the singleton {$\alpha$}, so $\alpha$ is not random. Formally this is done as follows. Suppose a sequence $\alpha$ is computable. Consider the following Martin-Löf test: for all $k$, just output the prefix $a_k$ of $\alpha$ of length $k$, and nothing else. This has measure at most (in fact, exactly) $2^{-k}$, and the intersection of the sets $U_k$ as in the definition is exactly {${\alpha}$}. QED!!

In fact a Martin-Löf random sequence $\alpha$ is incomputable in a much stronger sense: if some oracle computation with oracle $\beta$ (which itself is an infinite binary sequence) can compute $\alpha$, then for all $n$, $n-O(1)$ bits of $\beta$ are needed to compute the first $n$ bits of $\alpha$ (this is in fact a characterization of Martin-Löf randomness, which unfortunately is rarely stated as is in the literature). 



Ok, now the "edit" part of Joseph's question: Is it the case that a TM with access to a pure source of randomness (an oracle?), can compute a function that a classical TM cannot?

From a computability perspective, the answer is "yes and no". If you are given access to a random source as an oracle (where the output is presented as an infinite binary sequence), with probability 1 you will get a Martin-Löf random oracle, and as we saw earlier Martin-Löf random implies non-computable, so it suffices to output the oracle itself! Or if you want a function $f: \mathbb{N} \rightarrow \mathbb{N}$, you can consider the function $f$ which for all $n$ tells you how many zeroes there are among the first $n$ bits of your oracle. If the oracle is Martin-Löf random, this function will be non-computable. 

But of course you might argue that this is cheating: indeed, for a different oracle we might get a different function, so there is a non-reproducibility problem. Hence another way to understand your question is the following: is there a function $f$ which is non-computable, but which can be "computed with positive probability", in the sense that there is an Turing machine with access to a random oracle which, with positive probability (over the oracle), computes $f$. The answer is no, due to a theorem of Sacks whose proof is quite simple. Actually it has mainly been answered by Robin Kothari: if the probability for the TM to be correct is greater than 1/2, then one can look for all $n$ at all the possible oracle computations with input $n$ and find the output which gets the "majority vote", i.e. which is produced by a set of oracles of measure more than 1/2 (this can be done effectively). The argument even extend to smaller probabilities: suppose the TM outputs $f$ with probability $\epsilon >0$. By Lebesgue's density theorem, there exists a finite string $\sigma$ such that if we fix the first bits of the oracle to be exactly $\sigma$, and then get the other bits at random, then we compute $f$ with probability at least 0.99. By taking such a $\sigma$, we can apply the above argument again.