I haven't read all the posts so forgive if I say something that has been mentioned, but having a dd who switched from thumb to dummy (and will suck her thumb in absence of a dummy), I don't find dismissing with the dummy that much easier, as I loathe the idea of leaving her to cry when she needs her comfort.
Anyway what I wnated to say is that thumb sucking children tend to do it less as it impaires their play (you still have both hands free with a dummy), and also they tend to take it out when speaking, while I ahve to constantly remind dd that I can't understand her if she speaks with her dummy in her mouth, while I noticed most thumsucking chidren just take it out when thay want to play/talk.
Just a thought...but then again, in many cases they make the decision, and there's nothing you can do about it. Just be happy she has found a way of comforting herself, the aughter of a friend of mind likes to rub her mum's ear when in distress, that's something I'd definitely hate!!!