Check my answer on your other question about the legs. It applies (but I'll spare you the long lecture I gave before BUT IT STILL APPLIES)
Like Jarret said perfectly, yes, it is definitely attractive to see a girl's cleavage, but also other types of shirts are attractive. It's more than just cleavage/boobs that a man wants to see. But yes, seeing a girl's boobs or seeing an outline of them (you could be wearing a shirt that covers it more. ), or seeing outline of bra, or. Etc. Definitely attracts the attention of MOST men (not necessarily all, since were all different, and some don't value boobs over other assets or for whatever reason aren't as interested in them. )
So to sum it up, yes seeing a girl's cleavage usually attracts a man's attention and gets him interested, but longer shirts/different shirts of different styles/materials can also turn a man on.
Obviously it attracts guys. But what kind of guys do you want to attract. Do you want a guy that is interested in boobs, or someone that is interested in you as a person. Sure, what guy doesn't like seeing cleavage. But There's a difference between what we like seeing, and what who we want to date.
Too much reveal turns me off. Nothing left to look forward to! I would rather see a little bit of lace peeking out or just a bit of skin. I am not into a belly bulging over jeans or a lot of cleavage. I'd rather search for it myself than have it shoved in my face.