You should definetly look at the mobiBLU B153. Cnet got this thing to play 146 hours on a single charge! It has a "repeat all" mode and a "random all" mode. It also uses USB. It comes in 512MB, 1GB, and 2GB versions and is suprisingly cheap.
There site has a lot more information. The link is below.
I'm looking for an inexpensive MP3 player that I can set to SHUFFLE (or repeat all) that can be powered from 110V AC (either with an AC Adapter or USB power) that will just keep playing forever. I'll use it for a Music on Hold input for my phone system.
Any suggestions? (256Mb to 1GB is fine)