It's not as easy as the childproof doorknobs, but worked better for me in the long run. I've "bed trained" three kids this way so far. For the first few days after moving them out of the crib, I stand outside their door with my hand on the doorknob after putting them down for a nap. When they try to open the door, I hold the door knob in place so they can not turn it. I don't make any sound, so they don't know I'm there. They get mad and keep trying for a while the first couple of times, but eventually they give up and get back in bed, or sometimes they fall asleep on the floor in front of the door. But after a few days, maybe a week, they stop trying. Of course, they can get out just fine after the nap.
M.