One of the false accusations theists like to level at atheists is that we are just as faithful as they are. They insist that no amount of proof would change our point of view. Bullshit. I'd set aside the fact that theists have no proof but that is, in part, the point of this post. Other atheists have talked about what would constitute proof but I'd like to add a few more.
The first scenario I would accept as proof of God's existence is a simple message conveyed in a not so simple way. A message along the lines of, "I am God, I exist," would be sufficient provided it was delivered simultaneously all over the world in every possible language and medium for no less than an hour. Every written media (newspapers, magazines, books, billboards, etc.) spontaneously becomes re-written to contain that one message. Also all TV, cable, satellite, phone, radio, and internet venues carry the message. For good measure, it should also be broadcast telepathically. If God exists I see no reason this should not be possible. In the same token I cannot conceive of how an individual or group of individuals could possibly pull off such a message. Creating and coordinating a message in such a wide variety of media for even a short space of time would be virtually impossible. Re-writing and altering already created content in even one media would be impossible. And, since there is no such thing as telepathy (this avenue of message delivery could be shorter to avoid long-term harm) there could be no human agency involved. Group delusions have occurred but never on such a massive scale. It is also highly unlikely that after a half hour or more that a decent chunk of the Scientific community would not think to record the message in various ways. The skeptic/atheist/free thinker community would also probably seek to record it in various ways. It would then be verifiable beyond reasonable doubt.
I would accept that as proof. Anyone care to hold their breath waiting for this one?