This may sound like cruel and unusual punishment but it worked for a friend of mine. She had done the same thing and left the cot in the room. First she took the cot away and left the mattress on the floor. Each night she would put her DS into bed and he had the option of getting up later and being on the mattress. About a week later the mattress disappeared and she heard him get up and went in and put him back into bed and that was it. It seemed to be very much an out of sight out of mind thing after that.

A couple of other things you could try are putting the cot quit on the bed so that it feels and smells a bit more like normal or even moving the bed to where the cot has been.

Good luck.