What are the best current lower bounds on 3SAT? What are the best current lower bounds for time and circuit depth for 3SAT?
As far as I know, the best known "model-independent" time lower bound for SAT is the following. Let $T$ and $S$ be the running time and space bound of any SAT algorithm. Then we must have $T \cdot S \geq n^{2 \cos(\pi/7) - o(1)}$ infinitely often. Note $2 \cos(\pi/7) \approx 1.801$. (The result that Suresh cites is a little obsolete.) This result appeared in STACS 2010, but that is an extended abstract of a much longer paper, which you can get here: http://www.cs.cmu.edu/~ryanw/automated-lbs.pdf

Of course, the above work builds on a lot of prior work which is mentioned in Lipton's blog (see Suresh's answer). Also, as the space bound S gets close to n, the time lower bound T gets close to n as well. You can prove a better "time-space tradeoff" in this regime; see Dieter van Melkebeek's survey of SAT time-space lower bounds from 2008.

If you restrict yourself to multitape Turing machines, you can prove $T \cdot S \geq n^{2-o(1)}$ infinitely often. That was proved by Rahul Santhanam, and follows from a similar lower bound that's known for PALINDROMES in this model. We believe you should be able to prove a quadratic lower bound that is "model-independent" but that has been elusive for some time.

For non-uniform circuits with bounded fan-in, I know of no depth lower bound better than $\log n$.