I had previously stated that you would lose all your data upon upgrading your phone.  While this is true, it is possible to get most everything that you had on there back again by syncing up with iTunes and clicking the Restore From Backup button when registering your phone again.

I want to take a second here to emphasize that this will *NOT* rollback your OS to 2.2.1, you are locked in to the 3.0 version, but it will put all your backed up applications and music and what not back onto your iPhone.

Of course all of these articles rely on the fact that you are a developer with appropriate access to the 3.0 SDK and OS.