Q: What was the theological significance?
A: In antiquity, a kiss on the lips was seen as transferring a little bit of one's spirit to the other person. You have a lot of early -- I kind of think of them almost as Greco-Roman Harlequin -- novels that speak of the kiss as this transfer of spirit. Christians modify it a bit, to suggest that when Christians kiss each other, they don't just exchange their own spirit, but also share a part of the Holy Spirit with one another. So the kiss is seen as a way to bind the community together.
This is news to me, although I can't claim to be well-versed in the ancient novel (can one be well-versed in a novel?) ... anyone know what this is about?