For the last few months I have been using the Firefox addon Certificate Patrol to watch for changes to sites' SSL certs - so as to know if I am being MTIMed, against which SSL otherwise provides no protection.
However, almost every time I visit either Google or any site which uses Google Analytics or Google's CDN for scripts, I get a warning about Google's SSL cert, and Certificate Authority, having changed. (I occasionally get this on other sites such as Facebook, but not so often.)
Why does their cert change so often? It's not every time, but it's certainly many times a day.
What purpose does it serve? It obviously makes it impossible to detect a MTIM, which is a pretty big disadvantage...