A little over a week after the official launch of iOS 5.1, Apple blocked the process of signing SHSHs for iOS 5.1. This means that you can no longer restore to iOS 5.1 unless you have a SHSH saved for that version of iOS, and if you have not saved a SHSH then there is no method by which you can reinstall iOS 5.1 at the moment. It was normal for Apple to block the signing of SHSHs for that version of iOS since iOS 5.1.1 has been available to us for some time and I warned you to save your SHSHs early.
If you have done this and want to restore to iOS 5.1, then follow this tutorial if you have an iDevice released before iPad 2, and this tutorial for iPad 2, iPhone 4S and iPad 3.
