M.Z.
My daughter transitioned to a big girl bed and went many months before she started getting out of bed. I happened to be watching the SuperNanny show and this was one of the issues she had the parents do...I tried it and it worked great...very much like what Kristen L said (return him to bed, reduce your talking, ...)
The first time the child gets out of bed after you've tucked him in, carry/place him back in his bed and gently say goodnight and sit a few feet away from his bed, your back to him. 2nd time and thereafter, place him back in his bed, no talking, reduce your eye contact, and with each time you have to put him back in the bed, sit a little further away from his bed, keeping your back to him. Stand firm; there will be crying, maybe some screaming, but he'll get it quicker than you think.