I'm figuring that eventually they're going to allow 3rd party development, because 1) they made a point of saying it runs OS X, 2) because of the touchscreen, it can have any interface, for anything that could fit in your hand comfortably, and 3) what better developer community could they possibly enlist?

There'd have to be an awfully good reason not to use the programming stuff they already made, and alienate their salivating developers. Give Apple a little time; they're probably still finishing the final tweaks and parts sourcing. There might be rules for how to get in, but I'll be really surprised if there aren't ever any 3rd party applications for the iPhone.