I am not ticklish anywhere, although there is a bit around a scar on my abdomen which, like Dave S, can catch me out if I am not expecting it.
Isn't it a willpower thing, being ticklish? I refuse to be ticklish, so I am not. I refuse to feel the pain during a 5 hour tattoo inking session, so I do not (and my tattooist is amazed that I can have somewhere like an elbow or armpit inked without flinching at all, and she can tell when people are faking it to look hard as they always do a small flinch they cannot control.)
Willpower? Hmm.. I don't know about that.. Sometimes you don't want to be tickled, and are so anyway. And for the tattoo thing.. I think that's more of a strength/experience thing than anything else. I don't know as I have never had a tattoo, but I just think if you know what it's going to feel like, and you're strong enough.. you can defend against it somewhat.
It's always been my understanding that ticklishness is an automatic nervous reaction to the sudden pressure and/or motion hitting the nerves. Based on this, I'd say it's going to happen to anyone unless they've either got nerve damage, or are somehow capable of filtering out the nerve impulses entirely, as Happygeek seems to be claiming.