Developer Naughty Dog did make the multiplayer side free-to-play just over a year ago, but single-player going free for all would be something else. E3 is just around the corner, so who knows what the Californian studio has up its sleeve?
Update: Ladies and gentlemen, Nathan Drake has left the building. Sony has removed Uncharted 3's single-player from the PlayStation Store, so we're guessing - just maybe - it was a goof all along.
Uncharted 3: Drake's Deception (TGS 2011)