Technically speaking! it hasn't been even a complete day since Apple announced the availability of the iPhone OS 4.0 beta to developers; and the iPhone Dev Team have already managed to Jailbreak the firmware.
Only a couple of hours ago, MuscleNerd, a member of the iPhone Dev Team and master-mind behind most of the iPhone and iPod Touch Jailbreaks posted a tweet to update the users on the status of the Jailbreaking scenario for the new iPhone OS 4.0 and it seems that the future is bright.
The iPhone Dev Team, in only what seems to be less than 24 hours, the team managed to obtain root access on the firmware and install Cydia to showcase a foolproof Jailbreak achievement. Also it may be noted in the video that a VNC app is running in the background which further credits their claim.
As of now, this may only be seen as a start of what's more to come in the near future. Hang on and keep your fingers crossed!