The iPhone Dev Team recently released version 0.9.8b1 of its redsn0w tool, which added support for jailbreaking the first iOS 5 beta. Unfortunately for Windows users at the time, the release was for OS X only due to the fact that Apple had only released an iTunes 10.5 beta for Mac users. Now, however, the team has released a Windows version of the tool to accommodate the new Windows release of the beta iTunes version.
The jailbreak that can be accomplished with this tool remains tethered (so users must always plug their iDevice into their computer to achieve a reboot), as an untethered jailbreak will likely not be released until the iOS 5 Gold Master hits the public this fall.
You can read more about both releases at the team’s blog post.