yeah, the stuff you put on her thumb is not going to deter her. my mom used it to keep me from chewing my nails, but you end up figuring out that after a little while it wears off.
you know - in all honesty, i would try to put the smallest amount of pressure on her as possible. kids do stop on their own, and its a big deal. always remember, as dr sears says, an unfulfilled need turns into an undesireable habit later. this is a need for her to comfort herself. left unfulfilled, ..... who knows?
what i would do is give her the most control over it that you can. i dont really believe in sticker charts or stuff, but shes asking if she can suck her thumb, - encourage that somehow! :D try to do other things when she is tired or shy or uncomfortable that will distract her from sucking her thumb. maybe now is the time to let her pick a special toy or book or soemthing she can play with instead. bring that out only when shes going to be uncomfortable or tired. make sure you spend plenty of time snuggling her when shes tired. that might fulfill a comfort that shes getting from sucking her thumb. reading a book or 2 or 3 at bedtime might help too. (you might already be doing this - i dont know)
the less pressure you put on her, the more she will be able to confidentally move past this. shes at the age that if you just tell her to knock it off, she will think you dont understand her need, and she will most likely just do it in secret - which wont solve anything.
anyway, good luck.
try checking out www.askdrsears.com for some advice there. im sure that he will have something to say about it! :D
maybe distract her with gum? does she chew gum? LOL. a sucker or vitamin? i dont know. lol my son is more into the pacifier thing - i figured theres a point where you can take the pacifier away - the thumb is there to stay....