MuscleNerd just announced the release of RedSn0w 0.9.5 Beta that jailbreaks firmware 4.0 Beta 1 for the iPhone 3G. MuscleNerd states that the jailbreak is using same pwnage2 DFU exploit that’s been out for a few years now. Don’t go near it if you use yellowsn0w, ultrasn0w, or blacksn0w for an unlock. 4.0 will lock your iPhone and there is no available lock yet. Also you will have to be part of a developer team to activate the iPhone as this does not do hacktivation. We still have a few spots left if you would like to beta test the iPhone OS 4.0. You can find more info here.
Please also note that this is only for mac as the beta is only intended for developers who have a mac. You can install 4.0 on a mac and the sync with windows though.
Remember this is for iPhone 3G only (for now). No luck for iPhone 3G[S] users yet.