UPDATE : Since iPhone 3.0.1 is a minor upgrade, it is still jailbreakable and unlockable. Please stay away from 3.1 firmware when released, it can be a disaster otherwise!
All you have to do is :
1) Update your iPhone to 3.0.1 firmware via iTunes
2) Use redsn0w to jailbreak. While using redsn0w – when prompted to choose the firmware, simply choose 3.0 firmware instead of 3.0.1 firmware and it should work just fine! This works with iPhone 2G, 3G and 3GS.
3) Enjoy your iPhone!
As usual, stay away from the update for …