So you want to install the beta? Well, I have. Here are my suggestions (read them all the way through before starting):~

  1. Update to iTunes 9.1 first.
  2. Download and install the beta version of the Developer Tools package.
  3. Restart your computer. The installer won’t tell you to do this, but trust me, it may very well save you a lot of time and trouble.
  4. Start up, go into iTunes, and backup your device. Unless you’re okay with a clean slate (no old data), in which case don’t even open iTunes. Once complete, close iTunes completely.
  5. Open the new (beta) version of Xcode.app. Go into the Organizer, and then select your device (plug it in if you haven’t already). Select “Other Version” from the software version pulldown menu, and then hit Restore.
  6. You’re done!

Note: Not all of these steps are absolutely necessary, with a little luck. But by doing them straight away, you avoid a lot of possible issues.