Chris Iona talks about "Hacking the chumby". Has a control panel. Runs Adobe Flash Lite 3.0. Plays FLV, PNG, GIF, MP3, some streams. 350MHz ARM9. Can boot from cramfs and chroot a distro(Debian) from an external USB drive. The 2009 model has an internal battery.
More interesting stuff at Bunnies blog of chumby hacks.