For some reason, this text was the subject of several recent sermons I've heard -- you can read the whole passage here, along with various commentaries here. Several points are key: The woman recognizes Jesus as a Jew, and he recognizes her as a Samaritan, by their clothing; Jesus compares water that the Samaritan woman is drawing with "living water," which one, having drunk, will never thirst again. Depictions of this scene are common in early Church iconography (all of it later, of course, than that in the Syrian baptistery).
Peppard thinks this might be the earliest "securely datable" image of the Virgin Mary; he points out that the Annunciation was said to have occurred when Mary had gone to fetch water from a well (a scene also shown, though less often, in early Church art).
But I'm not convinced. For one, it's in a baptistery -- surely a place where the references to "living water" would have special resonance. For two, even at this early date, I think Mary would have been shown with some sort of halo. The fact that the figure is shown alone, to my mind, doesn't shift the argument one way or the other, and as for invisible angels, I'm not convinced. In this early period, before the great conflicts over iconoclasm, it seems to me that anything and anyone important would be shown, at least in symbolic form -- in any case, there's neither Christ nor angel here. But still more convincing, to me, is that this woman is not wearing any kind of veil or head-covering, as a good Jewish woman of the era would; my understanding is that, among Samaritan women generally, such coverings were not commonly worn. She also seems to have bare feet -- it's hard to imagine an artist of this era drawing Mary in such a fashion! It's intriguing, certainly, but I don't think that this argument is compelling enough to declare this the earliest depiction of Mary -- though it may be one of the earliest of the Samaritan woman!
