In an email, Apple has announced to its developers that all new iPhone App submissions to the App Store must now be iPhone OS 3.0 compatible.
Below you can read the text version and screenshot of email that developers received.
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.