Good point. I think his private PGP key and moving some coins around might be pretty convincing though... Even a stupid version of Nakamoto would understand that private keys are kept offline and in safe storage, especially if you made software as complex as Bitcoin. I think I would have thrown away my wallet so that the early mined blocks simply could not be moved even if I wanted to.
And in any case, Nakamoto may not be a single person after all.
The identity could remain unverified for eternity if original keys aren't available, and it's possible there was a deliberate destroying of the keys. Satoshi has said that the losing of coins were a donation to everyone else -- and who else would make a donation to the entirety of the Bitcoin ecosystem other than Satoshi?
And in any case, Nakamoto may not be a single person after all.