We're really eager to see what developers are coming up with to leverage all those great 3.0 features as well. Really, really eager. Ahem.
Full text of Apple's email to developers:
All apps must be compatible with iPhone OS 3.0
Millions of iPhone and iPod touch customers will move to iPhone OS 3.0 this summer. Beginning today, all submissions to the App Store will be reviewed on the latest beta of iPhone OS 3.0. If your app submission is not compatible with iPhone OS 3.0, it will not be approved.
Existing apps in the App Store should already run on iPhone OS 3.0 without modification, but you should test your existing apps with iPhone OS 3.0 to ensure there are no compatibility issues. After iPhone OS 3.0 becomes available to customers, any app that is incompatible with iPhone OS 3.0 may be removed from the App Store.
Begin testing now
iPhone OS 3.0 beta 5 and iPhone SDK 3.0 beta 5 are now posted to the iPhone Dev Center. Start testing today to ensure your application runs on iPhone OS 3.0. Visit the iPhone Dev Center for additional development information including iPhone SDK Release Notes for iPhone OS 3.0 beta 5 and Getting ready for iPhone OS 3.0.
(Thanks DevX for the tip!)