iOS 5.1 has finally dropped, and for the most part we totally recommend staying away from the update if you care anything about your jailbreak, as does the iPhone Dev Team.
That being said, for those with older A4 devices, it is possible to enjoy a tethered jailbreak on iOS 5.1 and still downgrade back to previous firmwares if you have your SHSH blobs for those firmwares.
With A5 devices like the iPad 2, and the iPhone 4S, it is impossible at the moment to downgrade to any older iOS 5.x firmware, even if you have your SHSH blobs saved. So don't upgrade to 5.1, especially if you care about your iPhone 4S jailbreak.
If you have an iPhone 4, iPod touch, iPhone 3GS, or original iPad, then check inside for a tutorial on how to perform a tethered jailbreak on iOS 5.1...