Ugh!!! I knew this would happen!! DS never wanted to take a paci and I was so worried that he would become a thumb sucker because of it and sure enough - he is a full-blown thumb sucker. When he gets really quiet - I know it's because he has his thumb in his mouth sucking away. It wouldn't bother me so much except I have 2 friends who's children were thumb suckers since birth and now their kids mouths look weird and stick out and the teeth are all crooked.
I tried so hard to get him to take a paci figuring that it would be easier to get rid of the paci (although not fun) when it was time than it would be to get rid of his thumb! Now when I see him sucking his thumb I gently take it out of his mouth and say "no thumb" and try to replace it with something else to suck on - but it's not working. He just laughs at me and does it again.