Install iOS 5.1.1 on iPod touch 4th gen not jailbroken with current iOS 4.1
Whenever an iOS device is restored, it checks with Apple's servers to see if the version requested is still being "signed". If it is, the restore goes as planned. Right now, the only iOS version being signed by Apple is 6.1.3.

There is currently no way to restore to iOS 5.1.1 without being jailbroken in the past.