and it worked!
This morning when I put Jonathan down for his nap, he fell asleep in my arms like normal, and also like usual he woke up and screamed as soon as I put him down. Only this time, instead of just figuring he'd have to cry himself back to sleep, I went right back in (about two seconds later, if that!) and told him "no, Jonathan, it is time to go to sleep" and lay him back down. He tried to pop back up a few times, and each time I just lay him back down with a firm "no, it is time to go to sleep." Then I got a pillow and lay down on the floor next to him. He cried a bit (very half-heartedly though!) and popped his head up three or four times just to make sure I was still there, but then he went to sleep!!! And after awhile I got up and left, and he stayed asleep. :)
I'd been wondering how in the world he always seemed to know the instant I walked away - now I think maybe he just wasn't really asleep yet, and was still in the "check to be sure she's still there and everything is ok" mode. It is really uncomfortable to hang over the side of a pack-n-play, so I think I was just leaving too soon. But lying on the floor next to him is quite comfortable, and I don't mind doing that for five or ten minutes!
So. Yay for new ideas, particularly new ideas that work! :)
Isn't parenting the most glorious and frustrating experience of trial and error?