-It does, of course, also depend on which MP3 encoder and settings you use; I did a most interesting (to me, anyway!) series of ABX trials on my own ears a few years ago.
From a 24/96 PCM master I prepared lots of transcodings - 16/44.1, 12/44.1, 8/44.1, 16/22, 12/22, 8/22, 320, 256, 192, 160, 128kbps mp3 with various options for encoding, ditto AAC - etc, etc.
I then used the (brilliant!) ABX plugin for Foobar and an excellent set of headphones, and after a few days I had a fair idea of how good my ears really were. Or, as it turns out, weren't. In most cases, 256kbps mp3 and red book were impossible to tell apart; 320kbps being indistinguishable in all but one case, if memory serves. However, at both 256 and 320kbps I had to listen hard for encoding artifacts rather than just enjoying the music.
Most tracks were eminently listenable at 192kbps mp3, but I could then reliably tell whether I listened to an mp3 or PCM without much effort.
Hard disk cost being what it is, I still ripped all my CDs lossless, though.
128kbps? I shudder at the thought of my 20-year-old-self telling myself and others that it was just about as good as uncompressed audio...
Oh, and you'd be surprised at how little dynamic range is actually utilized; on most tracks, telling 16/44.1 from 8/44.1 was next to impossible.
