The February issue Electronic Musician contains a "shootout" of epic proportions, one of which I've always wanted to perform myself. The article, Showdown at the Clubhouse, pits real amps versus their software counterparts and tests the ears of a panel of producer/engineer/guitarists. How'd they do? Well, not well is putting it nicely with a 38.5% accuracy choosing the real amp.
EM was nice enough to put the test online as well to allow anyone to take it. I took the test thinking it would be a breeze, but I was sorely mistaken scoring terrifically bad. However, consistently terrifically bad and incorrectly picked Line 6's Amp Farm a number of times. Go check it out and be sure to let me know how well (or poorly) you do...