Thumb Sucking: Your Advice

This is my boy.  As you can see, he sleeps with a little blankie-bear and he sucks his thumb--our own little Linus.  He began sucking his thumb as a baby, which, at the time, seemed really convenient since we no longer needed to worry about misplaced and dropped nuks.  If he wanted comfort, he simply popped his thumb in his mouth as required.  That was 5 years ago and my philosophy all along has been that he would stop sucking his thumb when he was ready.  I do not believe you can or should try to force this sort of thing on a child.  

Recently he has begun to develop an awareness that most kids his age do not suck their thumbs--he brought this up to me one night and we then spent some time discussing that he could stop sucking his thumb when he wanted and I would be happy to support him in whatever ways would help--so he made a decision to stop sucking his thumb basically during the day or in front of other kids, all on his own initiative, without any further help from me.  That was several months ago and he no longer sucks his thumb during the day--but does still at night, or when he is really tired.  

Then, on a recent visit with our dentist, as she was examining his permanent teeth coming in, she told me that she believes he will have perfect teeth (like his mama, I might add) that will not require orthodontic work--that is, if he stops sucking his thumb and soon.  His thumb sucking is already impacting his teeth growing in now and she encouraged us to work with him to stop.  

So, of course, Beeper and I discussed the fact that sucking his thumb may damage his beautiful new big boy teeth and that he might require braces later on and that it would be better if stops now, blahblahblah.  And he is such a conscientious child that with no further discussion, has since been trying not to suck his thumb at night--but the fact is that he just cannot seem to fall asleep unless he pops that damn thumb in his mouth.  So while he has reduced the amount of time he spends sucking it during our bedtime routine, every evening he will finally turn to me and ask, "Mommy, can I suck my thumb now?" (as if I personally have imposed a thumb-sucking moratorium) --and then once his thumb hits his mouth, he is out in 5 minutes or less.  Of course, I then slyly try to slip his thumb out of his mouth--which sometimes works, and other times he locks his arm up with the force of a world champion arm wrestler (in which case I leave it)--and I am sure his thumb is going back into his mouth during the night anyway....

So, my question to all of you mamas out there with thumb-suckers of your own, how have you handled this?  Any suggestions for how best to deal with it?  Is there anything you have found that works to support a child through making this sort of change in behavior and habit?  I hope it is clear that I am looking for a gentle approach to this--nothing punitive, nothing that will upset him--but I do want to help him and encourage him as I believe he has already decided for himself that he wants to stop sucking his thumb, but just cannot get past the falling asleep hurtle...so, give me your best advice.  I am eager to hear what you have found, tried and what you know.