If you’re like me and enjoy tinkering and playing with the latest iOS betas simply for the ‘fun of it’, chances are you’re pretty ‘cluey’ about iOS and how the iPad, iPhone and so on work.
As of the iPhone 3GS and iOS 3.x, Apple now signs every single firmware with a signature, unique to every single device and firmware called a ‘SHSH blob’. This means that usually once Apple releases an update, you can’t go back down, you’re forced to go up. Presumably in an attempt to prevent jailbreaking, to enforce security and to make sure everyone has the latest version.
Usually it’s pretty easy to downgrade if you’ve got the SHSH blob for your device for the firmware you’re trying to downgrade to. Unfortunately, as of iOS 4.2.1, Apple is making things somewhat trickier.