I asked my Rebbi this question, and he was unsure of a definitive source.
The Mishnah on Sotah 7b, (orig.) implies that actual cleavage (and below) is issue, as the Mishnah says, after tearing the Sotah's garment, to tie a rope "above her breasts". Rashi explains "so that her clothing not fall to the ground", implying that it would be unseemly to have her exposed from that point downward. Maimonides is more clear, (Hil' Sotah 3:9), saying "that her clothes not fall to the ground, leaving her naked".
Another area of the chest is mentioned in the Mishnah as well on 7a, (original), called "her heart". The Sages say to uncover that and to loose her hair, but Rabbi Judah asserts that "If her heart was beautiful, he would not uncover. If her hair was beautiful, he would not loose." On 8a, (orig.), this argument is examined, and there is a unanimous concern of arousal from this area.
So there is a area of a woman's chest1 that is deemed provokative, called her heart, that is above her breasts. A stringent, machmir interpretation of that passage would understand that area to start at the top of the chest, with the collarbone2 3.
1. I felt I could reasonably assume that the Sages are referring to the front of the woman's chest due to use of the word heart, which leads us there as opposed to the chin, neck, or sides.
2. Even without a formal source, 'communal standards' have halachic bearing.Ketuboth 72a, (original), penalizes a woman with forfeiture of her prenuptial agreement for transgressing 'Das Yehudis', laws of a Jewish woman. The mishnah gives details of this, listing specific outfits and behaviors. Rashi adds that D"Y comprises "customs of the daughters of Israel, even though they aren't written [in verse]". D"Y thus takes on a broader scope, depending upon community standard, within the parameters of the Mishnah. Maimonides on D"Y, (Hil' Ishus 24:11): "And what is Das Yehudis? They are customs of modesty practiced by the daughters of Israel... Maimonides too adds a general framework of D"Y, to the specifics of the Mishnah. Shulhan Arukh Eben Haezer 115 takes an identical stance. So general customs become halacha, concerning modesty. So regardless of the source, in communities that adopt a certain practice, it is halacha.
3. The collar bone is clearly not a black an white, be all end all line. the Sages aren't explicit about what area of the chest they mean, the Rishonim tell us that in general it really depends, and her heart is not listed as absolute D"Y. This could explain the discrepancy from community to community, as far as this area of dress. Also, The Tur, (ibid), does not give an explanatory preface, and just lists the Mishnah's items and explains. He does mention toward the end of this section that the prenuptial agreement was only intended for modest women, and not for the licentious, but that is much less conclusive than Rashi or Maimonides, though one could still draw similar conclusions.