I've been conducting listening tests as part of my dissertation, and two young individuals who show general acuity/ skill across all previous tests were able to identify mp3's about two thirds of the time. They were tested 30 times, using 10 different samples of music, each encoded at 3 bit rates: 128kbps, 192kbps, and and 256 kbps.
Neither of them did better at 128kbps, they both did best at 256kbps. I would have expected/ hoped they'd do better at the 128kbps bit rate.... it looks like 128kbps is actually really rather good!!