iFaith, the SHSH dumping tool, has received an update and is now available to the public for download.
Up until iFaith came out, if you wanted to restore your device to a previous iOS firmware, the only way to do it was to save the current firmware's SHSH blob using tools such as @notcom's TinyUmbrella or Cydia's 'Make my life easier' function which automatically saved your SHSH blobs to Cydia's server.
If you didn't save the SHSH blob and Apple stopped signing for that iOS then you would not be able to downgrade, as iTunes would stop the process from being completed during the 'Verifying firmware' phase of the restore.
The unique thing with iFaith is that, regardless of what firmware your device is running, iFaith will extract the SHSH blob directly from the device. That way, you'll to be able to restore back to that same firmware in the future, even if Apple is not signing that firmware...