Erica Sadun, iPhone hacker extraordinaire and writer at The Unofficial Apple Weblog, has documented the entire set of Cocoa function calls required to program for the iPhone. These header files are used for programmers to properly create user interfaces, network code, and, well, pretty much everything. And the documentation effort is a massive job, usually not something to be done by just one person. I know that this site can get kind of wonky here and there, so I'll do my best to explain why this is important, but for everybody.
All of the applications available from Installer.app have been written without any formal set of documentation. So, there may be some bugs, since there's no single place to go for programming information. Usually, Apple would provide the documentation for programming on the iPhone. But, as they've recently announced, they're not going to be doing that until February. So, now anyone that is planning or writing a native Cocoa app for the iPhone or iPod touch now has the means to research how to do it.
That includes both the folks that are hacking iPhones to install and write 3rd party apps, and any larger software companies that want to get a leg up on their software development. With this set of header files, it should be perfectly possible for any large development group to prototype their program well in advance of the official Apple release.
p> Of course, these header files may yet change. There's no guarantee that Apple's set of documentation will stay the same; Apple will definitely be adding to this, and they may not allow some of the function calls documented by Sadun to be accessible for other programmers. No one can tell. But, it's a huge step for programming native applications for the iPhone and iPod Touch