I would guess that very few people can tell the difference between a 256 or 320 mp3 and a cd especially not on average equipment.
Actually, the differences between MP3 & CD are easily heard on very modest equipment.
Hi-Rez 24/96 files can sound amazingly real. Coming soon are 24/192 downloads.
I have all the Beatles albums on CDRs burned direct from the master tapes. I expect Apple Corp are determining the best vehicle for Hi-Rez 24/96 downloads and simply delaying decision on the iTunes MP3 format.