All you have to do is download iTunes 9.2 for Windows (avail. as of yesterday), and find a torrent for iOS 4 GM (3GS). You don't need a registered UDID, you don't need to jailbreak. Also, if you shift+click "Update" instead of "Restore", it works fine. I've done it two years in a row for the GM builds.
And yes OP it's pretty damn awesome sauce. Once all the third-party apps are recompiled in the OS4 SDK we will have something truly awesome on our hands. For now, you can tell if an app is 4.0-compatible when it has a "+" icon. (the same + as iTunes+). In my experience though, one + app I tested; "Farm Story", did not support saved states. Don't know what that's about but we'll see.