Keep in mind I haven't read any definitie spoilers, but while hanging over at TVTropes, I came up with something that... honestly seems to be fairly watertight.
Pay attention while watching The Rebel Flesh. The Doctor gets analyzed by the Flesh, then he goes outside and his shoes get melted by the acid. Which might seem silly at first, but remember, Gangers copy every cell, including clothing. Therefore, the Doctorgänger will be wearing the old shoes (and will be at least in that regard identifiable from the real Doctor.
But it's bigger than that. Now go back and watch The Impossible Astronaut and look at the Doctor's shoes. They are his original shoes, not the replacements. Which means that the Doctorgänger (with all the Doctor's memories intact) is the one getting shot on the beach, not our Doctor.
Which leads me to the conclusion that the Astronaut... is the Doctor.
Witoki 16:29, May 26, 2011 (UTC)