Robin Givhan of the Washington Post has an entertaining article about Hillary Clinton showing cleavage recently. Givhan has some strong views on the matter of cleavage and how it pertains to presidential candidates. She also notes;

Showing cleavage is a request to be engaged in a particular way. It doesn't necessarily mean that a woman is asking to be objectified, but it does suggest a certain confidence and physical ease. It means that a woman is content being perceived as a sexual person in addition to being seen as someone who is intelligent, authoritative, witty and whatever else might define her personality. It also means that she feels that all those other characteristics are so apparent and undeniable, that they will not be overshadowed.

To display cleavage in a setting that does not involve cocktails and hors d'oeuvres is a provocation. It requires that a woman be utterly at ease in her skin, coolly confident about her appearance, unflinching about her sense of style. Any hint of ambivalence makes everyone uncomfortable.

See her entertaining cleavage article here.