It is not even out yet, but the good people over at the iPhone Dev Team have managed to Jailbreak iOS 4.1. ?Comex, the creator of the Spirit and jailbreakme.com jailbreaks is saying that he has already located 2 exploits in iOS 4.1. The first one is similar to the jailbreakme.com jailbreak as it uses a loophole in iOS itself, which makes it simple for the user to impliment, whereas the second one is in the bootrom of the iPhone at the hardware level. Which makes the second jailbreak will be much harder to impliment than the first, but as the second one is deep down within the iPhone, it will be much harder for Apple to lock it down.
Look for it to be available the same day that iOS 4.1 is, which is rumoured to be September 8th.