Here are the official download links to iOS 5.1 directly from Apple:
- iPhone 4S
- iPhone 4 (GSM)
- iPhone 4 (CDMA)
- iPhone 3GS
- iPad 3 WiFi
- iPad 3 GSM
- iPad 3 CDMA
- iPad 2 WiFi
- iPad 2 GSM
- iPad 2 CDMA
- iPad
- iPod touch 3G
- iPod touch 4G
- Apple TV 2G (5.0)
- Apple TV 3G (5.0)