What is the medical significance of a widow's peak?

There isn't much..: It is just a cosmetic finding. It used to be thought to be an omen of early widowhood. (Old wive's tale) Widow's peaks are seen in a couple of rare genetic syndromes, Donnai-Barrow syndrome, Waardenburg Syndrome and Aarskog syndrome are the ones I can think of. But they have other associated anomalies along with the widow's peak. Really rare.