I agree that there is such a thing as dressing inappropriately. Let's further assume that they are doing so for the express purpose of attracting your attention. (In my experience, men are usually deluding themselves about how much women dress to seduce them, but whatever).
The question is, how are you going to react to it?
Because one person dresses inappropriately, do you have to react to it like a 14-year-old boy? Are you going to comment on it in a way that implies that women are there for your personal entertainment? Are you, by opening your mouth, going to encourage others to get even cruder? And what about all the other women you work with, or will potentially work with (who probably resent the woman who overdoes the cleavage too)?
I understand that you feel that it's unrealistic to ban sexuality from the workplace. But it's not prudishness that is called for, but judgment. Consider that your actions take place in an existing context. In 2012, it doesn't take too many comments like that before your workplace or your open source project or whatever turns into a hostile space for women.
Now, I've worked on volunteer projects with people who knew each other really well, and things are considerably looser with regards to sexual humor.
The point is, you have to create an environment where women have a secure status. In the typical workplace, people barely know each other, and women are just barely holding onto whatever status they've earned. This probably means refraining from the more puerile jokes and observations.
P.S. In the interests of full disclosure, I've failed at this sometimes myself. I had a colleague who was a great programmer, but I was distracted by her body. She wore shapeless t-shirts covered in shapeless hoodies most of the time, in part, I'm sure, to hide this. But more's the pity that she basically has to dress in a tent in order to have her colleagues look her in the eye.