We've seen videos of iPhone 4.0 Beta being jailbroken, but we thought we'd have to wait for quite a while until the jailbreak tool is released. Well, surprise! It's already available for iPhone 3G devices.
The Dev Team has made the redsn0w 0.9.5 BETA download available now, but they do caution you:
Please note that this beta is not meant for the average end-user. There are many things "broken" with jailbroken apps in the 4.0beta1 environment right now [...] This beta redsn0w allows the developers behind those jailbroken apps (like MobileTerminal.app!) to fix their software before the general public gets iphoneOS 4.0.
Because it's meant for JB app developers, this beta redsn0w does not perform hactivation. You'll need a properly-registered developer UDID with Apple to get past the activation screen. For similar reasons, there is no Windows version of this beta redsn0w (since apps are developed on MacOSX). Please don't pirate Apple software.