This was already reported on Engadget and Slashdot, but in case you missed it; Rockbox has been ported to the iRiver H100 series! The bad news: No realtime MP3 playback (yet). The good news: Rockboy, a port of the gnuboy gameboy emulator! It's still a little rough around the edges, so I've started submitting patches. The first patch — which has already been accepted to CVS — adds a menu to Rockboy, and the second patch adds load and save state support (via the menu).
Anyway, Rockbox is great. It boots up about 3 times faster than the stock iRiver interface, and it has a lot more features, too! If you're an H120 owner who's unfazed by flashing firmware and cross compiling, then I highly recommend you give Rockbox a try (once the MP3 playback is ironed out, that is). As my parting gift, here's Bionic Commando running on my H120 (and a couple shots of my menu system as well):