The Modal Ontological Argument

I've been paying attention to this argument lately, mostly thanks to a discussion with Peter Lupu about the problem of evil. Here's the basic argument:

1 God is the greatest possible being (GPB).
2 The GPB is one that possesses all perfections necessarily.
3 Necessary existence is a perfection.
4 It is possible that the GPB exists.
5 If it is possible that the GPB exists, then that being exists necessarily...[thus, GPB aka God exists]

Let's revise premise 3 to only say that "existence is a perfection." Now, what's the problem here? If God's perfections accompany him in all the possible worlds where he exists, then we already know that existence accompanies him in those worlds. Ahh, but for all we know there are no possible worlds where God exists. So we definitely need premise 3 to stay as is.

Let's revise premise 2 to say that "the GPB is one that possesses all perfections." If God's perfections may/may not accompany him in all the possible worlds where he exists, and God has the perfection of necessary existence in merely one possible world...doesn't he exist in all possible worlds?  Yes, but then again, how do we show that there is a possible world where God exists and also happens to possess the property of necessary existence?  It would seem that this premise is more driven by our concept of God.  Our concept of a GPB seems to demand that he possess all perfections necessarily, and not merely contingently...since presumably a being who possesses perfections necessarily is better than one that doesn't.

Hmm, so it seems like the argument does indeed need to be exactly as stated.  If we can show that God possibly exists, we have shown that he actually exists.  Much more to think about, but a very interesting argument at first glance.