iH8sn0w, developer of sn0wbreeze and iFaith, has found a way to downgrade the firmware on A5 devices.
Using saved SHSH blobs, iH8sn0w was able to downgrade his A5 iPad 2 from iOS 5.1 to iOS 5.0.1. He says his method will also work with the A5X processor once firmware updates are released for the new iPad. That means that it's possible to restore to any firmware you want on A5 devices, as long as you have the SHSH blobs saved.
This is great news for those with the iPhone 4S and newer iPads. As it stands now, if you have to restore for any reason, you will be forced to upgrade to the latest firmware. This could leave you without the ability to jailbreak for some time.
Found a loophole in Apple's apticket system. Restored my iPad 2 to 5.0.1 from 5.1. Works on all A5 devices. :)
A5X devices will work too once Apple pushes a new firmware for it.