Apple isn't playing nice with the jailbreaking community and has recently started blocking devices from restoring to older firmware. If you accidently updated to 5.0.1 and want to restore your device, hacker iH8sn0w has a new solution for you. iFaith v1.4 allows you to save your iOS device’s SHSH blobs which should allow you to downgrade your device in the future.
Unfortunately it doesn't support A5 devices. Meaning if you recently purchased an iPhone 4S, or upgraded to iOS 5.0.1, you're still out of luck for now. A4 owners are being urged by the jailbreaking community to download the latest update of iFaith and to save their SHSH blobs immediately.