As expected Apple has stopped signing iOS 5.0.1 with the release of iOS 5.1. If you own a jailbroken device you should not update to 5.1 because it breaks both the Corona untether and the Absinthe exploit. The Dev-Team warned both veteran and amateur jailbreakers about the dangers of upgrading on Wednesday.
iPhone 4S and iPad 2 users who did not heed the warning won't be able to downgrade back to 5.0.1 even if they did save their SHSH blobs. People with earlier devices and saved SHSH blobs can downgrade with the assistance of geohot’s limera1n exploit. There is also a tethered jailbreak for non-A5 devices if you really have the urge to get to 5.1.
I would suggest avoiding 5.1 entirely since it doesn't offer anything new for the iPhone. For more info and download links visit the Dev-Team blog.