Playlist's review of USB speakers is well done. I ended up liking the description of the Creative TravelSound Notebook 500, but the price is over $80. For that much I'd like to see them first. I like using iPod speakers powered by the USB cord -- less to carry!
(Note these are not USB speakers, they are USB powered! I need them for my iPod, so I don't want to waste money on an unused D/A converter.)