Windows/Mac: The iPhone jailbreak tool Redsn0w has been updated this morning to support a tethered jailbreak for the just released iOS 5.1.
We've talked about the pros and cons of a tethered jailbreak before, but it means once you jailbreak, you have to plug your iPhone into a computer if you want to reboot. Many jailbreak tweaks aren't ready for 5.1 either, so use this only if you really want to jailbreak your recently updated device. The process is the same as previous jailbreaks, but you need to download the new version of Redsn0w (Windows/Mac).
The jailbreak works on all non-A5 devices (it doesn't work for iPhone 4S and iPad 2) and if you have an iPhone 3GS it's completely untethered.
March Mayhem [Dev-Team Blog]