rusleeping: I hadn't heard of the best paper session before, but I will definitely check it out if it takes place at this conference.
A lot of regional conferences don't have them, as they tend to be smaller (obviously!) and have fewer sessions, but ocassionally they do.
I can't ever remember a national conference that didn't have Top Paper sessions in most divisions. Sometimes they just note in the program with a * that X paper won top faculty paper or top graduate student paper. But often there will be a "Top Paper" session. I go to them not just to see the writing (if they make copies available), but to see what the "best" academics are doing.