iOS 5.1. OTA Update Problems
Some users are experiencing problems when updating their devices via OTA, ie through the device itself without connecting to iTunes. If you iOS reports the following error when updating: “It is impossible to find an update, an error occurred while looking for the software update.” The solution is simple and can be done from your own device.
Follow the following path: Settings – WiFi – Click the blue arrow on your WIFI network
Once here, we have to change the numbers in the DNS, putting the Google Public DNS: 188.8.131.52.
Now, if you go to Software Update, yes you would have to be able to update via OTA.
VERY IMPORTANT: do not do this if you have done jailbreak, if you will have to update via iTunes. OTA update a jailbroken device may corrupt the files inside the device and leave it unused.