I ran into some problems with the 4.2.1 iOS update. After the update iTunes presented me with the error 1013 popup and sent my iPhone into (permanent) recovery mode. Being an ex jailbrek user, I remembered a tool called iRecovery, that could bring iPhones out of recovery mode after a 10XX error. And while googling for it, I found an app called RecBoot, that does the same, but with GUI. Sure enough, I've downloaded it and a minute later my iPhone was out of recovery and updated to 4.2.1. Miracle! So if your iPhone gets stuck in recovery after the update, just give RecBoot a try. For clarification's sake, I'm using a non jailbroken iPhone 4 and Mac OS X 10.6.5. And I have no clue what so ever, why I had issues with the 4.2.1 update. Never before have I expirenced iTunes behave like this!