Even after several firmware releases, it has always been possible to implement Geohot’s limera1n exploit which is associated with the hardware on non-A5 devices. Just in time when Redsn0w is updated, Sn0wbreeze now does a tethered JailBreak for iPhone 4, iPhone 3GS, iPod touch 3 and iPad on iOS 5.1. This tool is entirely dedicated to work on Windows. So if you need an alternative of Redsn0w, this should be the right choice. 
All those users who rely on ultrasn0w should make a custom firmware and restore it in pwned DFU mode in iTunes. Needless to say, ultrasn0w is not yet updated to unlock iDevices on iOS 5.1. That’s why, you may need to wait for a while. As always, the JailBreak is untethered only for iPhone 3GS users with an old bootrom. All others would need to connect their devices to the software once the device reboots. If you fail to do so, cydia apps including mobile safari wouldn’t work.
Moreover, if you have updated accidentally to iOS 5.1, there’s no way out to downgrade with the saved SHSH Blobs. Apple is no more signing the old firmware, iOS 5.0.1. This version of Sn0wbreeze doesn’t support A5 devices such as iPhone 4S and iPad 2.
You can download latest version of Sn0wbreeze from here (windows only).