is "spaghetti sort" really O(n) (even as a thought experiment) ? I`m referring to the notion described here: http://en.wikipedia.org/wiki/Spaghetti_sort

In the analysis section the author admits that considering it to be O(n) requires the assumption that the act of identifying and then removing the longest spaghetti rod which
has stopped your hand from descending further is an O(1)  rather than an O(n) or O(logn) operation - this seems to me a rather unreasonable assumption especially if many rodes of relatively simillar height exist. 
Alternatively it seems equivalent to assuming that one can pick the longest rode from a heap of rodes arbitrarily spread on a table at O(1) without going through the "leveling" procedure at all.
The folklore analysis of spaghetti sort assumes a model where one can extract the longest noodle in constant time, perhaps by lowering a horizontal plate ("hand") over the vertical noodles until it stops, and then extracting the noddle touching the plate.  (Apparently the plate is pastamagnetic.)  Within this perfectly well-defined theoretical model, the analysis is clean and obviously correct.

So is the pastamagnetic-plate model reasonable?  For up to a few hundred strands of spaghetti, sure.  Beyond that, obviously not.  So should we make the model more reasonable by taking the physical costs of moving the plate, sensing collisions, or  choosing the right noodle?  What about the (nonzero!) physical cost of keeping all the noddles vertical (or at least parallel), and keeping the sensing plate and the table horizontal (or at least parallel to each other but not the noodles)?  Should we worry about the measuring process causing micro-fractures that change the exact length of the noddles, or the implication from quantum mechanics that there is no such thing as "exact length"?  Holy crap, is the spaghetti-sorting problem even well-defined?!

No, this way lies madness.  The original theoretical analysis is predictive within the context for which the algorithm was designed — manually sorting a handful of pasta #8.  That the analysis does not generalize to significantly larger values of $n$ or other types of pasta does not make the analysis "incorrect".  It merely limits its scope.