The wikipedia page on kindle has a bunch of neat facts about what formats work and where you can get more content.

Seems like the Kindle's PID (product ID used with its DRM) was reverse engineered which has allowed the use of encrypted mobi format books to be read on the Kindle. Otherwise the Kindle only supports non-DRM mobi titles. Kindles own format is a variation of mobi.

Fictionwise has used this information and now serves up their mobi book catalog in Kindle-compatible versions. So there are a few places to get books from besides Amazon.

I'm not sure what it's going to take to make the ebook reader as ubiquitous as the portable MP3 player, but it's not going to be this generation of device/platform. It's a market I'll keep my eye on though. If I weren't interested I wouldn't have started the thread after all.