*Jimbobob hops onto this train late in the game*
You know, that may have something to do with why pictures of girls in candid situations with amazing boobs does so much more for me than posed models (usually, but not always). Something about a girl downplaying or, as mentioned here, attempting to hide the social power and status that large breast may endow her with is really attractive to me. Some girls put them out on display, showing off their power. The ones more up my alley tend to have that bit of embarassment (or possibly awe?) at their endowments.
Maybe it's that idea that they're hiding something. I mean really, half the reason we're obsessed with breasts is that we can't see them, and men are obsessed with what they can't see. Perhaps it's that primal, low level brain activity urge to get what we want when we want it, and being denied it repeatedly and constantly drives us nuts. As a fan of hard-to-get girls with personality and spunk (as opposed to the easy, give it up if you ask nicely girls), I think the explanation works for me.
Or perhaps that's just me.