oh wow, what an interesting debate this question sparked! Thank you for all your replies, it's been really interesting, and I must admit, my point of view seemed to change with every post!

I've since tried DS with a dummy and he sucks happily for about 20 secs then it falls out. So i'm leaning towards letting him thumb suck after all. Interestingly the time he's most likely to do it is after his first bottle in a feed (and yes, AJC, still the left hand!) but not much at other times. So I think we'll just keep our options open!

I must admit, as a child I had a thumb-sucking "fluff" routine. I'd pick bits of fluff off my bright red blankie, and hold it in the crook of my index finger (above the thumb in mouth) and brush it back & forth above my upper lip (below my nose) as I sucked my thumb. I loved it - go figure huh! I think my parents were more frustrated by the red fluff that pervaded the house than my teeth (oh and occasionally the red fluff disappeared up my noise too! ). So that's my dirty little thumb secret tehehehe.