In the 20 some cruises that I have sailed with them I received one upgrade and that was only due to the fact it was 2 weeks after 9-11 and they were upgrading everyone. RCI explains it as once you have chosen an assigned cabin your chances for an upgrade are almost none. Who knows for sure? I always book the cabin and category that I'm happy with and if I do get an upgrade it's only that much better.
I'm hoping for an upgrade as well. Cruised Celebrity in March and started with an inside cabin and wound up with a rather nice outside cabin (window only). I keep an eye on the prices and if they start going down I have my agent check for an upgrade. My guess is if you aren't upgraded before arriving at the ship, it isn't going to happen.