Re:Language? (Score 3, Informative)

The Kindle application framework is Java based. You write "booklets" that work like Java applets. Under the hood the Kindle runs a Linux kernel, so in theory you could just write native C apps, but I doubt Amazon will give developers access to that.
Some more info about hacking your Kindle:

http://igorsk.blogspot.com/2007/12/hacking-kindle-part-3-root-shell-and.html