Well folks...sweeping the Internet and the media is that latest theory that Tony was indeed "whacked", presumably by that dude who walked past him on the way to the men's room upon his return.
Evidently, the theory is that the screen goes blank mid-scene was set up the episode before, when in a rowboat Tony is discussing with Bobby, who conjectures that "everything just goes blank" when one gets whacked.
Producer David Chase apparently is non-committal about this explaination, but has supposedly intimated that this sceanario is a "very good theory".