As a body language signal: When she's in front of a man, a woman either crossing her arms over her chest, or placing her arms or hands in front of her chest while topless is a sneaky way for her to draw attention to her breasts as Cynthia Brimall demonstrates (NSFW). It's alleged to be a gesture of sexual anticipation laced with anxiety.
Of course, there are many other body-language experts that say crossing one's arms is a gesture of rejection, defensiveness and/or hostility, so, take all such research with a grain of salt.
Besides, as women concerned with a certain level of modesty know, when things are showing that you didn't really want to show, because it's cold, if you know what I mean... there are a limited number of options, and crossing your arms on your chest is one of them.
There is a bit of a legend at Oxford. As a formerly all male institution, there is a short section of the Cherwell that has been fenced off for men to bathe in the river. Naturally, only the oldest and most esteemed professors, still quite fond of the old traditions, use these facilities. When the fence came down in a flood one day, the professors were too busy to notice. However, when a boat carrying several young female students went past, the professors flung their hands to shield the ladies from their most private of parts. One professor, however, flung his hands to his face. When asked about it later, he said, "I don't know about you all, but I'm known by my face around here."
The same story has many versions around the world, often involving clergy. Look to the jokes for an example.
