As I suspected, Apple plans to release an SDK for native apps on the iPhone next year. The official word is on Apple's Hot News site. They appear to have pre-announced the SDK which is likely to have its official announcement at MacWorld in January, and they say it will ship in February. I'm sure that this was the plan all along, and the developer frenzy has managed to flush out a pre-announcement to give some guidance to the market, which I think is a good move at this point.
There has also been speculation that the iPhone "runs Leopard". I think that the truth is likely to be a bit more subtle than that. There has to be a master code base for MacOSX, and efficient engineering management practices should try to minimize branches in that code base. The master code base has a release branch called "Leopard" and another one called "iPhone", but the differences between them should be as small as possible. Bug fixes and features get checked back into the master code base whenever possible. So the work put into Leopard is leveraged for the iPhone, and we should see some of the feature set that was announced for Leopard transfer to the iPhone.
I think there is a good chance that Apple will release iPhone 1.1.2 immediately after they release Leopard, and that some Leopard features will be included. One obvious feature would be localized versions of Mail and other apps for the iPhone and iPod Touch, bringing the iPod Touch more in line with the iPhone's application set. There is a new and upgraded iChat in Leopard, and it would also make sense to release this for the iPhone/Touch.
I also noticed that the DTrace analysis/monitoring framework from Solaris 10 is included in Leopard. This is very cool, it provides a huge upgrade in observability for performance analysis of both desktop and server applications.