So the whole Nikki Leotardo theory is bullshit. That's too bad, because it would have confirmed what I think was going on. Not that I really need it confirmed. I'm pretty comfortable with my reading, which is this:
Questions about whether Tony "lives" or "dies" after the final scene miss the point, which is that there is no "after." First let me back up to say that my initial reaction was that of course he's dead -- not because the black screen represents his consciousness or anything that some literalists are insisting on, but because the narrative cohesion of the scene demands it. But then I realized that the abrupt ending was a way of intentionally breaking narrative cohesion, so something else is going on.
Even before Chase said he wasn't trying to "(mess)" with people, I didn't buy the "prank on the viewer" theory. It just doesn't fit the tone of the series. Rather, I think what Chase tried (successfully) to do was to rewrite one of the basic principles of storytelling, which is that a story has to have an end.
Robert McKee says there are only two types of endings: closed and open. Closed endings leave "nothing in doubt, nothing unsated." Open ones leave some questions to be answered by the audience, but "open doesn't mean the film quits in the middle, leaving everything hanging. The question must be answerable, the emotion resolvable."
Chase doesn't do that either. Instead, his non-ending deliberately suspends the action. It does not project forward to one possible ending or another (Tony might die, Tony might go to jail, Tony might take over New York). It creates a permanent state of suspended action — Tony exists forever in a single scene of ambiguous but deadly menace. (This is essentially a more postmodern version of Alan Sepinwall's Theory #1.)
Now when I first heard that all the characters in the diner were people who had been on the show before -- and who had reason to want to kill Tony -- I thought my reading was confirmed. I know some people took it to mean that Tony was clearly about to be killed, but that doesn't work because there's no way all those people would "actually" be in one place all at the same time. They would have had to be symbolic of Tony's deadly karma. Even without this confirmation, I still think my reading makes more sense than any other. If Chase had wanted us to decide what happens "next," he would have used the traditional language of cinema -- a fade out or something -- to indicate that there is a "next" that we're not getting to see. By violating that, he's trying to say that there is not one. The show has just, as the song says, stopped.
Just because Chase was not trying to "(mess)" with viewers, doesn't mean that people conditioned by McKeean (or is it Aristotelian?) principles will not feel (messed) with. Here's McKee again:
All films need a Resolution as a courtesy to the audience. For if the Climax has moved the filmgoers, if they're laughing helplessly, riveted with terror, flushed with social outrage, wiping away tears, it's rude suddenly to go black and roll the titles... A film needs what the theater calls a "slow curtain." A line of description at the bottom of the last page that sends the camera slowly back or tracking along images for a few seconds, so the audiene can catch its breath, gather its thoughts, and leave the cinema with dignity.
I won't argue that this isn't usually true, but Chase broke the rule to great effect. The ending was probably the most satisfying moment of the entire season.
Update: All that said, I'm still open to the theory that Tony comes back to life with cleavers on his hands.