For a truly portable sound system on the go (about the size of a credit card with the thickness of a pack of gum) could we recommend IDEA International's Card Speaker? It's the Robin to your iPod's Batman, available in either black or silver, and offers portable playback for up to 5 hours on a 2 hour charge via USB cable. And like the 1st generation iPod, the speaker's backside is mirror polished so you can use it as mirror when you pull it out of your purse, backpack or laptop case.
The IDEA International Card Speaker is available for $75.
[via Better Living Through Design]