From everything that I've read, the iPhone has some problems with applications. If Apple can't even manage to get their own apps to function without crashes, I really wouldn't want to see how unstable third party apps would make the iPhone. That being said, this is iPhone OS X 1.0. I'm sure that Apple is working on a way to have a better protected app space, so apps can be sandboxed. Once they have that and it's been debugged, I'm sure that Apple will release an official SDK. In the interim, Apple has pointed to Web 2.0, so that they don't look like they're shutting developers out entirely. After all, we all know that Apple doesn't comment on future products... until it does.
Anyone remember what Apple had to say about dual booting Windows on the early Intel Macs, right up until they released BootCamp beta?