There was a new iOS release 6.1.1 yesterday which fixes some cellular issues with the iPhone 4S. I have one of those, but haven't noticed any problems. I did apply the update yesterday and everything's fine.
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If you have a different model of phone, did you get an update notification? You should see something in your settings icon and you should also get it when you connect your phone to iTunes. The explanation of the update says that it fixes some issues with cellular on the 4S. It doesn't seem to do anything else. You can probably ignore the update unless the notifications annoy you. I'd imagine you can apply the update without any worries if your phone is a different model, though.