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.