I have played around with the iPhone SDK and Apple has done an amazing job. For starters, the documentation is impressive in both quantity and quality, including a series of introductory videos (quite the contrast with the scarcity of API documentation when the Facebook API first became available). The SDK provides access to most but not all of the nifty features of the iPhone, including the camera, accelerometer and of course the gestures on the touch screen (dock access and bluetooth are not available as far as I can tell). Right now user interfaces for native apps have to be created programmatically, which is painful, but the Interface Builder will greatly simplify that (the version of the Interface Builder included in the Beta of the SDK does not support the iPhone yet). But amazingly it appears possible to create scripted applications and deliver them over the web that will be nearly indistinguishable from native apps. This is due to a number of extensions in the iPhone browser, including the ability for a web app to create a button on the phones home page, operate in full screen mode, store persistent local data in a DB and make use of advanced CSS features (still figuring out what determines if the app itself stays cached). If that turns out to work as I think it does it takes away a lot of the constraint of app delivery only via Apple (for native apps). The SDK also includes a great simulator for the iPhone, which is important given that actually deploying to the device for testing is a bit painful – I believe Apple has made this hard intentionally (requires code signing certificate from Apple and a bunch of device prep) to prevent this from becoming a back door for distributing native apps.  Here is an interesting and fairly detailed scorecard for the SDK.