CNET's blog One More Thing reports that Apple the App Store based on the new iPhone SDK and firmware 2.0 would already launch as early as Monday.

The source for the rumor is a developer at a software company working on 3rd party iPhone applications.

Apple's way to control applications developed with the iPhone SDK is to distribute them only through the App Store that will be accessible through iTunes.
Of course if the developer charges money for the iPhone application, Apple takes a share of the revenue (30%).

Theoretically Apple could release the App Store and the new iPhone firmware 2.0, without yet shipping the expected 3G iPhone. Existing iPhone users can upgrade to the new firmware and start using official 3rd party iPhone applications.
More details on CNET.