one of the reasons holding me back from getting an iPhone originally, was that it was a closed system. i would not be able to write apps for it, or download other 3rd part apps. but all that changed when apple announced they would in fact release a Software Development Kit for the iPhone in Februrary. Well, they were only a little late releasing it today. joy. And it's just as sexy as everything else Apple makes. very slick.
http://developer.apple.com/iphone/program/ however, and i don't know why i didn't guess this in the first place, you need an "Intel-based Mac running Mac OS X v10.5.2 and later". that's a problem for me, with only WIntel and Linux systems on hand. Now whom do i know with Macs on hand? :)