Yep, got this message again this morning. I called OnStar and they suggested that I remove the rubber bottom from the storage compartment in the top center of the dash, right above the center stack display, and insert the tip of the key on the remote into a small hole I would find there. Supposedly this would resync the fob to the car. Hmmm... well I did that and I don't know if that worked or not, but I was able to get the car to take me to work. I called my dealer from work and told them about the issue and so I have an appointment tomorrow morning to have them check it out.
I had almost no trouble coming home. The remote wouldn't unlock the car at first, until I got right up to it... usually I can unlock the car from a football field away. Yes, I already thought of the battery being low and had replaced it a couple of weeks ago. Anyway the car did unlock eventually and it didn't argue with me about powering up and driving home. Once home, I opened the door and then closed it again and I got the message about the missing remote. That seemed to repeatedly trigger it. For fun, I opened the fob and took the battery out, wiped it down and put it back in. The doors promptly locked themselves and I got no further message about a missing remote. Hmmm... could it be that simple? I will still take the car over to the dealer tomorrow.
I asked OnStar if they would be able to get the car "started" for me if I ran into this on the road somewhere, and they said no but they could get me a tow truck. Seems like all the neat stuff you see them doing via OnStar on TV would include bypassing the missing remote thing too. I bet they really can... :-)