This is quite significant, as it was previously only possible to downgrade to a 4.x.x firmware and not an earlier 5.x.x firmware. Downgrading is used in a case where you have accidentally, or out of necessity, upgraded your device to a newer firmware that cannot be jailbroken. In this situation, you would need to downgrade your firmware to an earlier version, and re-jailbreak. You could use Sn0wbreeze or the Pwnage Tool to create a custom firmware to restore with in iTunes. Once iOS 5.0 was released, it was discovered that this method would no longer be possible, especially on devices that are launched with an iOS 5.x.x firmware installed. iH8sn0w says this will most likely work on the iPad 3 as well, once an update to iOS 5.1 is released.
The loophole iH8sn0w has found, in the apticket system, which involves your device’s SHSH blobs. So, just as before, if you want to downgrade your firmware, you’ll need to have your SHSH blobs saved for your device, on the earlier firmware you wish to downgrade to. You can use notcom’s TinyUmbrella to easily save your SHSH blobs with one click.
iH8sn0w has also said that this does NOT apply to the baseband, so this will be of no help to you unlockers out there. Unfortunately, once you have upgraded, you’re out of luck.
There’s is no eta for any tool or software that iH8sn0w may put together for this, but we’ll keep you updated.