I noticed that some of those who jailbroken today via jailbreakme.com are confused about the possibility of decoding. At the moment ultrasn0w decodes any baseband from iOS 3.x up to iOS 4.0.1, but the baseband must not be higher than 05.13.04, i.e. the one from iOS 4.0.1. If you have baseband 05.13.04 or something lower, you can quietly decode with ultrasn0w from Cydia. You also have a decoding tutorial here.
