You already know that starting with iOS 5, Apple modified the restore process through iTunes and SHSHs saved in Cydia or with TinyUmbrella became unusable. When the announcement was made regarding the impossibility of restoring to iOS 5, the Dev Team announced that it would look for a solution to allow us to use SHSHs for restoring, and a new version of redsn0w will allow this. MuscleNerd announced today that a new version of redsn0w will be released and it will allow the use of SHSHs saved with TinyUmbrella or Cydia.
Since everything is done through redsn0w, I assume that this will be the program through which a custom ipsw must be made, but we will find out more when the program is published. Until then, I recommend you to get the latest version of TinyUmbrella available here and save a SHSH for your iDevice if you haven't done it yet. With iOS 5.0 you will not be able to restore unless Cydia has any SHSH saved, but with 5.0.1 you should be able to do it in the future.
