Here is an interesting demo I heard about on This Week in Media.  It contains 2 audio clips in both 128 kbps and 256kbps mp3.  One clip is Mozart and the other from R.E.M.  The Mozart clip has a lot of strings in it.  The R.E.M. clip has background vocals.  Both are where you might find the differences represented.  To be honest, I can't hear a clear difference.  The strings feel a little more mushy but that could be psychoacoustic too.  Certainly it's not the sort of thing I would notice without an A-B comparison.  Give the demo a listen.  Can you hear the differences?