In the first 4 months and over 3000 miles of driving, I have not found one thing that needs dealer attention. I am very pleased that something so new and high tech is also so perfect.
The only thing that happened on the first day, was my fault. I started charging it, then locked the doors, and then I decided to show the wife how to plug it in. So I unplugged it while charging and while locked. Well we all know what happens when we do this. Alarm went off thinking the cord was being stolen. I also got a check engine light because of this. I contacted onstar and they said it was do to the charging system getting interrupted while charging. They could offer no further details and told me to see a dealer immediately to make sure.
I did not go to the dealer because the light went away and I knew what happened. It never happened again, perfect after that. This tells me onstar may direct people to the dealer unnecessarily.
Another thing that happened. When backing up, I got the camera going and the sensors on. Well this time I was in a parking lot and while backing up, a car comes full speed around the corner and almost crashed into my rear. My Volt shut off and a message appeared saying to put it in park and restart.
This never happened again, so I figure it is a safety feature, but not totally sure.
2/5/2013 Update. I ran a search for other Chevy Volt drivers that have had their car for 2 years now. None of them have noticed any battery degradation so far even after 2 years.