Yes you heard it right, Yesterday only apple released iOS 4.0GM for developers and today it was Jailbreaked. This was done by msft.guy. He has posted PWNGAGE bundles (modified version of original Pwnage Tool) which jailbreaks iPhone 3GS iOS 4.0 GM, without updating the baseband. Also on the other hand Devteam’s MuscleNerd have updated that redsn0w 0.9.5 beta can jailbreak the iPhone 3G iOS 4.0GM but it should be legit one that means it is not relying on blacksn0w or ultrasn0w for network unlock.
To jailbreak iPhone 3G iOS 4.0GM you need to first update your iPhone 3G ‘s OS to iOS 4.0GM with iTunes 9.2 (available for mac users and developers only). Then on redsn0w 0.9.5 beta you’ve to point to OS 4.0beta4 IPSW instead of iOS 4.0GM.
But I would recommend all of you to just wait till 21st till iOS 4.0 out for public and by that time I believe that Devteam will also come up with their jailbreak and unlock tool.