Lots of people give their answers (including Rowan) here.
Doug's been writing some interesting stuff on this recently (here is the latest).
My problem remains how to reconcile Jesus' humanity with his special creation; or, put differently, I don't see why God's creative activity _has_to_ conflict with the normal processes of reproduction. Incarnation isn't dependent on it; indeed, I suspect that the story was developed in order to support the doctrine of the incarnation and now works to accomplish the precise opposite. Either way it's an extremely marginal belief and not essential to faith.