I just did it. Search for iTunes 10.5 Beta 7, which is the only one I could find for Windows. Then search for your .ipsw file. I don't know if its allowed to post links here, but they aren't hard to find.
I installed iOS 5 fine on both windows and OS X Lion, Windows i didnt have to change anything but with my Mac i had to change my HOSTS file and erase the gs.apple.com IP address..