With Jake, he wore the pull-ups to bed for a long time after he was potty trained. One night I accidently forgot to put one on for bed and he woke up dry. To make a long story short, with him it was the pull up...if he had one on he would just go in it, if it wasn't on he got up and went potty. We put a plastic matress cover on his bed and tossed the pull-ups. We had a couple accidents and still do occasionally(although now they usually happen because he just can't wake up enough to pull his pants down fast enough when he gets to the potty).
I agree with Tara, try leaving the diaper off, if he wakes up dry...REWARD!!! If he wakes up wet, clean it but don't make a big deal out of it, kids usually feel bad enough when it happens. Waking him up to potty once or twice for a little while is a good idea too, my DH did that before he went to bed(Jake had usually been asleep for a couple of hours by then).