Apple's iPhone SDK (Software Development Kit) is now freely available to anyone from the iPhone Dev Center. You're only required to register as an iPhone Developer, which is free.
iTunes App Store
With the release of iPhone firmware version 2.0 the iTunes application store will open. Apps will be available for both the iPhone and iPod Touch. Many will be free, but developers are also able to charge for their applications.
In order to test your code on an iPhone and make it available on the App Store, you must apply to the iPhone Developer Program. Then for $99/year for the Standard program and $299/year for the Enterprise Program you'll be able to test your code on actual hardware, get tech support directly from Apple and be able to publish your application via the App Store.
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