I don't know that it can be guaranteed, but I think that it is ususally not a problem. Even if they couldn't get you connecting rooms, they would probably at least be able to get you adjoining rooms.
I believe if there are only 2 adults in a large party, then they would guarantee the connecting rooms. Other than that, it is a request. They do try hard to accomodate.
We went with our 5 children, and had guaranteed connecting rooms. We were told that if the children outnumber the adults the connecting rooms are guaranteed.
I hate to be the negative response here but I feel connecting rooms have to be the hardest thing at Disney to get. We have requested them every trip because of a medical condition and each time we have had to beg and plead at check-in to get them. What I can't understand is I know people who have been placed in connecting rooms who didn't even want them. Why do they do that when they know ahead of time that there are guests coming in who have asked for them?
We were GUARANTEED connecting rooms at CBR this past April because we have 4 children..........they do not want children alone in a room without a parent. (Not that we would have done that but I guess some people would!)