I listen to everything on an iPod. If I have something on CD, I import it lossless, not because I can hear any difference but solely for peace of mind. I have about 5,000 tracks, roughly half of which are in 128k, which was the iTunes standard when I bought them. The other half are divided between 256k and lossless (imported off the CD). I can't tell any difference between the bit rates. While low-level stuff (64k) is obviously inferior in quality, I hear more difference between different recordings than I do between the higher bit rates.
What's more, I think headphone listening makes some of these comparisons practically moot. Confessing no perceived difference between "standard" and "premium" bit rates may make me vulnerable to the braggings of someone who considers 256k "slumming it," most of the loudspeaker music I hear in my daily life comes nowhere near the "slumming" I do with my headphones. I was just thinking about it, riding in my car, listening to the radio. It's not that my car's stereo is so bad, as car stereos go. It's just that the standards are so different. What passes for decent audio, within a moving vehicle navigating traffic, is so unbalanced and so lacking in clarity that my average Grado session just blows it away.