THEORY: Tony Soprano is dead.
EVIDENCE: The biggest clue is, of course, the sudden fade to black at the end, symbolizing the way "you never see it coming" and "everything just fades (or cuts) to black". Also, black is the color of mourning, also indicating death. Then there's the button on the jukebox that Tony presses, K4. That plus the blackness, or "nothingness", at the end of the episode, means that Tony was "Killed For Nothing". Also, in an earlier scene in the episode, Tony is barefoot and walking out of step with the rest of the cast, which is a symbol of death in many Italian cultures.
One piece of evidence most people overlook is the publicity shot taken to promote the episode, in which Tony is standing with his back to the camera, Carmela is holding her hand up over his head (more symbols of death), and Tony can be seen wearing a patch that says O.P.W., which stands for "Officially Pronounced Whacked". Also there is a bouquet of flowers in the background that appears to spell out "TONY?" or perhaps "TONY86", indicating he's been 86'ed.
Also, if you play the entire episode backwards, you can hear a number of verbal clues, such as the sound of gunfire and Tony screaming, a voice saying "Bada bing, dead man", another voice saying "Tony's dead, fuhgeddaboudit, miss him, miss him", and a third voice saying either "I whacked Tony" or perhaps "cannelloni".
Then, of course, there's the onion rings, the most blatant clue in the entire episode. I don't think I need to explain what those mean.