News from DevTeam:
iPhone 3.0 OS is jailbreakable on all devices. It’s just those using 3G yellowsn0w that have to show some restraint and wait for PwnageTool to create a custom IPSW that avoids the baseband update. If you are using yellowsn0w, do not update or restore to iPhone 3.0 firmware. You will lose yellowsn0w and find yourself unable to revert the baseband to get it back.