The iPhone SDK Agreement (3rd item under Downloads on the iPhone SDK page) almost very likely includes several limitations on use of the SDK, and an NDA, which one has to agree to when registering to download the iPhone SDK. There may be several requirements regarding any application which one is allowed to develop using the SDK. Read Section 3.3 carefully before asking if an app can be developed to do this and such.
The SDK is only officially supported on an Intel Mac running OS 10.5. It cannot run under Windows, as it is based on all the Mac OS X frameworks, libraries and XCode Developer Tools.
The difference between a registered and an enrolled developer:
Registered: register at Apple, agree to the SDK license, and use the SDK with the included iPhone simulator only.
Enrolled: Pay $99 and be allowed to provision actual iPhone and iPod Touch devices to test your software.
Commercial License: more legal forms to fill out after you are enrolled but before you are allowed to sell your app to others. Requires banking information.
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