iOS 4.2.1 now implements a baseband signature check in this update. Meaning if you have a unlocked iPhone and install the update, guess what? It won’t boot! Apple is now trying to prevent iPhone users from having the old baseband installed. If you have a unlocked iPhone, don’t update!!!! Instead wait for a unlock tool for iOS 4.2.1. If you have a unlocked iPhone and did update, well… you could always downgrade your firmware. I recommend NOT updating to iOS 4.2.1 if you have an unlocked iPhone. This doesn’t apply to iOS 4.2 if your wondering.