DS (13 weeks) is a thumb sucker. He has been doing it since he was in utero - we saw it on the scan (naw!) Personally I find it to be an absolute blessing - he can settle himself really easily and will often wake up and then go back to sleep just by sucking his thumb.
I sucked my thumb until I was 5, when my parents undertook an elaborate system of bribes to get me to stop. I had to have braces when I was 13, but not for an overbite - I had an underbite and still do.
TBH, I figure we're probably going to have to fork out for braces for DS at some point anyway (DH has pretty crooked teeth too) so the thumb sucking is only providing positive benefits at this point. JMHO though.