iPhone OS 2.1 beta 4 is now available and is to be used for testing only. Once you have updated your devices with iPhone OS 2.1 beta 4, you cannot restore them to an earlier version of iPhone OS. Applications developed using the beta versions of iPhone OS 2.1 cannot be submitted to the App Store.
View the Pre-Installation Advisory for iPhone OS 2.1 beta 4, Readme, and Release Notes before installing the new versions of the iPhone OS and SDK. As a reminder, pre-release software is Confidential Information and is subject to the terms outlined in your Registered iPhone Developer Terms and Conditions with Apple.