You can make a downgrade from iOS 5 beta if you saved the SHSH

MuscleNerd confirmed iOS you can upload to your iPhone 5 beta 1, test, and if for whatever reason want to return to iOS 4.x (and you have the SHSH stored in Cydia) to downgrade.

The only exception is the iPad 2, as there is no known exploit bootrom level is impossible to downgrades.

If you decide to upgrade to 5 beta 1 iOS remember that your baseband will be updated and you can not release or software or any card liberating.

via | iClarified