Up until now, only older iDevices have had the ability to downgrade to previous firmware versions using the Limera1n exploit and SHSH blobs, but that ability has now been ported to newer A5-powered iDevices like the third-gen iPad and the iPhone 4S.
This news comes by way of the iPhone Dev-Team, stating that redsn0w version 0.9.11b1 directly supports the restoration of older firmware versions to your jailbroken iPhone or iPad. There is no need for a bootrom-level exploit and the new feature can be found under Extras -> Even More -> Restore.
Of course, you’ll need the personalized SHSH blobs to be able to downgrade to that firmware; these blobs can then be remote or local for the restoration process. It’s also important to note that the A5 downgrade will update to the latest firmware before reverting back to the older one. What this means is that Apple can eventually close the exploit with a newer firmware, but they haven’t yet.
As always, proceed at your own risk and discretion.