Well, if nothing else it proves how easy it is to use the SDK.
This took five days from release of the SDK to the video of the demo going up, I reckon on another 95 days until the AppStore goes live, so plenty of time for developers of all shapes and sizes to get to grips with writing great apps.
One other thought, I haven't heard it expressed anywhere else yet (although I may just not have been looking hard enough), the iPhone is going to get a lot of developers used to programming in Cocoa that have never considered developing for the Mac platform before; I can see a potentially huge 'halo effect' of bringing developers, particularly games developers, over to the Mac as a platform via the iPhone.