Well, I just downloaded the iOS 5.0 IPSW file for my iPhone 4 and did the offline update through iTunes. It didn’t work, and I got a message “The iPhone could not be updated an unknown error occurred (3002). At first, I wondered-my iPhone is not jailbroken or unlocked also I use the latest version of iTunes that is compatible with iOS 5. After testing a few methods, I found a simple way to fix iOS 5 3002 error in iTunes and here is it.
Right now a lot of people are downloading/activating iOS 5.0 at the same time. So the reason for iTunes error 3002 is nothing but the failure in verification of your iPhone/iPod/iPad with Apple servers. You can easily fix this error is buy editing your Windows/Mac host file. Before proceeding to the next setup check whether your iTunes version is 10.5.0, then login to your Apple account and authorize your machine from the store Tab.
Follow the instruction depending upon your OS type and do it exactly as written. The process will bypass Apple verification for your device and thus it fixes the 3002 error. Now repeat the iOS 5.0 update method in online/offline IPSW restore mode. Whiteman08 has posted a few more methods in the comment section. Check that too!