I recently ran into a problem after upgrading an iPhone to iOS 5.0.1 where the Update and Restore buttons on iTunes are greyed out (not allowing you to click on either). I believe this problem is not related to the iOS version, but rather the network service provider. Usually if there are any setting changes to the network providers' network, you will receive a notice in iTunes when you plug in your phone the first time after upgrading, that tells you an update to the carrier settings is available and offers to upgrade it for you. If you do not upgrade, you may not be able to receive a signal and your iPhone/iPad may show "No Service" on the status bar.
Especially with iOS 5.x where you can activate your device without connecting to iTunes, this issue may happen more easily. Or if you are using an iPhone which is locked to another network other than the one on your SIM card, this could happen also. So what happens now that you have No Service on your iDevice and you are unable to even Update or Restore?
