When I'm upgrading my iPhone 4 from iOS 4 to iOS 5 this morning, I got a weird error 3002. After a short thoughts, I tried this solution and it's worked!
- Download and install iTunes 10.5 from http://www.apple.com/itunes
- Download iOS 5 package from http://iosindex.com/ios/iphone/4g-gsm/5.0-9a334
- Plug your iPhone to PC, and right click on the device name on iTunes.
- Choose "Transfer Purchases" from the list, to save you apps from iPhone to PC.
- Once it's done, click the device again and choose backup.
- Wait until backup is completed, press "Shift" on your keyboard and click on the "Restore" button.
- A window explorer will appear. Browse to the .ipsw file you have downloaded.
- Press "Open" and your upgrade will be started shortly.