I've always given myself some credit for having a good ear. My friend and I have done AB tests with our respective iPods (5th gen + 6th gen classics), and I was able to tell which was which every time. (not that hard - the 5th gen sounds a lot better [warmer, clearer]- unfortunately for me, a proud owner of the 6th)
I guess my pride in my ear was misplaced.
I was not able to tell the difference. Actually, that's not entirely true. I thought I could perceive a difference, but upon PMing chengbin, I was proven wrong.
This does not convince me to rip at lower bit rates, however. I still feel that my setup (sr-60 out of power mac g5) is not quite enough to be able to properly use as a reference, and I'd hate to have to re rip all my tracks.
(one last possibility - I might simply prefer the mp3. As a member of the "iPod generation [I'll just say under 20 years old, in my case], the compression might sound better. Hopefully that's not the case, but if you are under 25, and have listened with mp3s your whole life, post which you preferred )