I'd be surprised if your score didn't go up after paying off a mortgage, just for the simple reason that your total debt load and monthly payment totals will plummet. Seems like a formula for a score increase in my opinion, but I'm only guessing. Of course, if you then go and obtain a new mortgage on another home, then that could cause it to go right back down again.
Any mortgage experts out there wanna take a stab at this?