There is no official jailbreak utility for iOS 4.0.1. However you can still jailbreak and unlock iPhone 3G. Just do the following:

• Download the latest RedSn0w 0.9.5-b5 in our utilities page here ^[1].
• Download firmware images for iOS 4.0 here ^[2].
• Run RedSn0w and point it at the 4.0 IPSW. (Yes – point it at the 4.0 IPSW even though you're at 4.0.1). This will also hacktivate your iPhone3G at 4.0.1 if you haven't already been activated by iTunes.
• Install ultrasn0w in Cydia (add repository http://repo666.ultrasn0w.com) if you need a carrier unlock (and in the USA, remember to turn off 3G mode in Settings→General→Network).