D.D.
My 20 month old has been sleeping in a full size bed (with rails) for almost a month now. He took to it at night without a problem. As long as he is tired, he stays in bed. We don't have a problem with him getting up and coming back out to see us. Nap times were a bit different since it was daylight outside but now he is good with naps.
The first night, I laid down with him until he fell asleep. I was so nervous about him falling out of bed that I ended up back in his room around 3 am and slept with him. The second night, I laid with him for about 15 minutes until he drifted off. After that, he was good to go!
I know it seems early but for us, I knew that we were going to need the crib soon for our next baby so we started the transition early thinking it would take a long time to get him to the point where he would stay in the bed. It didn't but we might have some regression next week when the baby arrives. I figure I have another 6 weeks at least if he does regress since the baby will be sleeping in our room for at least 6 - 8 weeks.
Hope this helps.
D.