Joe Johnston of Chicago, commenting on the Times Online website, offers one of the best interpretations I've seen:
|The final moment wasn't ambiguous at all. You just have to think about how it ends and, if you account for a number of overlooked details--i.e., the elongated moment with the blank screen before credits roll, and especially the sudden cessation of the music mid-song--you'll see that there's nothing ambiguous about the ending.|
Tony is whacked. It happens at the very end -- but we see it from Tony's point of view, which is the sudden nullity of instant death from a gunshot wound to the head.
Another commenter adds that it recalls the episode where Bobby and Tony are sitting in his fishing boat, talking about hits. Tony remarks that you would never see it coming, the world would just go black.
I'm down with that analysis.