I would not use limerain if you plan on unlocking, as the way unlocking ability preservation works sometimes relies on older baseband. So if you're on 4.02 and upgrade to 4.1 to use geohot's limerain you might be stuck for a while. This is still good news for many people thoughNew jailbreaks are out as of today GMT (10 Oct)
www.limera1n.com
I can confirm it works on my week 40 manufactured 4.1 firmware iPhone 4.
Ultrasn0w carrier unlock still does not work (yet) on any firmware after 4.0.2 however.
Hopefully the dev team's jailbreak will be right after 4.2, as they've now postponed their release.