Modal Logic and the Ontological Proof

The ontological proof for the existence of God (really "proofs" or perhaps "arguments," as there are various versions) has popped up in the blogs a few times recently: e.g. Ophelia Benson, Josh Rosenau, Jerry Coyne. You've probably heard this one; it was most famously formulated by Saint Anselm, and most famously trashed by Immanuel "Existence is not a predicate" Kant. A cartoon version of it would be something like