While it is probable that most apps should already run on iPhone OS 3.0 without modification, developers are being encouraged to test their existing apps to ensure there are no compatibility issues. Apple will deal with any incompatibility issues swiftly according to the email: 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.
The letter concludes with the suggestion that developers not waste any time and start testing now with iPhone OS 3.0 beta 5 and iPhone SDK 3.0 beta 5. Furthermore, developers are encouraged to visit the iPhone Dev Center for important information about these new betas.
If you are testing iPhone OS 3.0 beta 5 and you have discovered any incompatible apps, please tell us about it in the comments.