There's an interesting discussion at FQI about Gaunilo's objection to Anselm's ontological argument. I think the objection is usually overrated. One reason (of several) is that it equivocates. Anselm's argument uses the following description:
(A) that than which no greater can be thought
It argues from this to the existence of (A). Gaunilo's objection attempts to build a parallel description that allows one to reduce to absurdity:
(B) that island than which no greater can be thought
But (B) is ambiguous. It can mean either:
(C) that island than which no greater island can be thought
And if we look at what people say about the objection, they almost always seem to take it in this sense. But (C) is not parallel to (A). The true parallel is:
(D) that island than which nothing greater can be thought
This is obviously problematic in a way that (A) is not, namely, that we can easily think of a being greater than any island.
More can be said about this matter, but I only have this terminal for a few more minutes. I'll say more about other issues that come up with the Island Objection when I'm at a campus computer.