MuscleNerd recently tweeted that redsn0w has just released the first public 4.0beta1 jailbreak (redsn0w 0.9.5 BETA), but most iPhone owners will want to hold off for the time being.
Here is the message he posted earlier today:
However, it is made very clear on the redsn0w website that this is not for the average iPhone user. The developers say that there are still many things "broken" with the apps in 4.0beta1 right now This software is geared towards those who develop jailbroken apps.
Currently, the software is only available to those with iPhone 3G (Not going to work for the iPad) and a Mac, because "apps are developed on MacOSX" as the developer puts it.
The good news is that DevTeam is hard at work for iPhone 4.0 jailbreak