I love the concept, but even with the freaky missing shoulder, I didn't see it as maybe being a torso til I read the copy. Really tough to pull off...

true and the lips in the top corner are a bit of a giveaway...

It's a nipple! Yes it is!

I'll admit at first I saw a just neck and thought "feh.. not post-worthy" and it hung around my desk for a while.. But then I caught it off-guard one day and saw a breast and a waist.. oooh.. now that's pretty neat. So I posted it. :)
I don't think the web is the best place for it, on a bigger sheet of paper when you aren't prepared the illusion can work. But it is very hard to pull off as you can see. Still, I fancy the thought.

looks like a tit to me

Over here at PDN online Eye On Ads: Visual Ambiguity Works For Lowe's Cancer Campaign they have more on how they got this image.

But could it be done? And on a shoestring budget? "I didn't know if we could do it," says Hogya, who, with high hopes, sent comps to photographer Frank W. Ockenfels 3. "Frank is acclaimed for his celebrity shots," says Hogya, "but when it comes down to it, he's a problem solver." Ockenfels was attracted to the challenge and the concept's sophistication. "It has a tremendously European flavor," he says. "It doesn't talk down to anyone, which a lot of American advertising does. We're talking a huge leap of faith that you can show a visual like this and people will get the point."