The iOS Dev-Team, the folks behind a lot of the Jailbreak technology, have issued an Admiral Ackbar-style warning to Jailbreakers and unlockers about the impending iOS 4.1 update -- stay away for now:

This time of year there are lots of new iPhone owners, and not everybody knows that accepting new iOS updates is the surest way to lose your jailbreak and/or unlock. While those of you who have Cydia or TinyUmbrella backups of your FW hashes will always be able to get back to 4.0.1 if you make this mistake, this doesn’t hold for unlockers. There’s currently no known way to revert your baseband — if you update your baseband you’ll lose the ultrasn0w unlock, possible forever.

On the plus side, sounds like a safe, ultrans0w-friendly Jailbreak upgrade path for 4.1 is in the pipeline. Maybe a clean 4.1 Jailbreak as well?