You nailed it. It's kind of easy to switch between Apple IDs and it SHOULD be as easy (or easier) to switch between Game Center accounts.
It didn't take long for me to get thoroughly frustrated when I was playing a game and then realized that my kids progress on the same game overrides anything I had accomplished. No way to go back and play levels I missed. No way to learn gameplay at less advanced levels.
Big problem if Apple thinks this is going to be a family (or even two person) gaming device.
In my opinion, there should be something on the homescreen (if there are more than one users) that can be easily accessed - a kind of "who's using this now" kind of feature. Or better yet, get rid of Game Center and just allow multiple user profiles on games!