Gavin & Craig met in person, and Gavin & Satoshi corresponded in private for many months. Who doesn't think a smart guy like Gavin would ask Craig questions that he knew only Satoshi could know, or someone who had 'access' to them at the very least. Craig is either Satoshi, or part of the Satoshi 'entity'. Many here don't like the implications of this, but it is what it is.
edit: e.g. 'what was the 15th word in the email I sent you on X date?'
It is not that easy. A lot of the emails were forwarded to other people or their mailboxes might have been exploited. You cannot rely on that type of information to validate Satoshi's identity. He just needs to sign a message in the genesis block and be done with it. Nobody can dispute that, if he does that. The question is, does Satoshi still have access to that? Satoshi had no idea that Bitcoin would be this successful, back then it was still experimental and he might have thrown that information away.
Did anyone consider that Satoshi might not have access to the PGP keys or the private keys and could be to embarrassed to admit it? < Imagine this headline in the media - The creator of Bitcoin lost the private keys to 1 Mil bitcoins! >
