iOS 5.0.1 your software is up to date while updated iPad

iOS 5.1 is available for download through an over-the-air update or by connecting your iPad, iPhone,or iPod Touch to iTunes. This software update includes improved audio, new podcast controls, a Japanese Siri, Camera shortcut and etc. When I try to update my iPad 2 to iOS 5.1, it just says "iOS5.0.1 Your software is up to date" or "Unable to Check for Update. An error occured while checking for