Note – this also works for the current iOS 5 beta if you need or else you can use the activation trick here to upgrade to the iOS 5 beta without a development account.

After installing the iPhone 3.0 Beta you’ll find that you’re unable to sync your iPhone without having your iPhone’s UDID listed on an Apple Developer Connection account. The only problem is, if your UDID wasn’t previously listed, there’s no obvious way to find it with the 3.0 Beta software installed on your iPhone.

On OS X you can use XCODE’s iPhone browser window to get the UDID. On Window’s it almost as easy:

Windows XP: Browse to C:\Documents and Settings\%username%\Application Data\Apple Computer\Lockdown

Windows Vista: Browse to C:\Users\%username%\AppData\roaming\Apple Computer\Lockdown

The file found in that folder is entitled with your UDID. Enjoy 3.0 :D

Chris

Update: This can be found on OS X in the following directory: ~/Library/Application Support/MobileSync/Backups