The iPhone Dev-Team have issued warnings to all you ultrasn0w users wishing to download the all new iPhone 3.1 Beta, we have already mentioned this earlier but this is a recap warning to let you know again.
The iPhone 3.1 is all good with new features etc but these warnings about jailbreak/unlock users and new firmware updates should be read carefully, the dev-team says that ultrasn0w users must stay away from official 3.1 including beta versions or you will lose the unlock for good.
So the bottom line is; do NOT download no matter how nice it all sounds, let the dev team test the tools out first and when they are ready they will let you know.
Source — The iPhone Blog