I've got nothing against ambiguous endings - the ending of Henry Fool in particular is wonderful (although if you see Fay Grim, the ambiguity is, obviously, utterly shattered). But this wasn't even an ending; this was a cocktease. (As one forum poster said: "David Chase, you get back to this bed right now and fucking finish me off!")
The diner sequence at the end was one of the most suspenseful scenes I can recall from the entire Sopranos run; everything is soaked in dread (well, except for the Journey) and, well, it's clear that it's THE LAST SCENE, and everyone is expecting SOMETHING to happen, even if it's nothing, just the family eating in a shitty diner, fade to black. Even the editing of that scene is wierd - Tony is at the door, then he's at the table, and he's looking at a jukebox filled with songs from the 1980s, which initially led me to believe it was some sort of flashback.
But by denying us even nothing - by delibarely fucking with us by thinking our cable went out - that's just a deliberate effort by David Chase to remind us that we're watching a television show, that he's bucking convention, fucking with the audience. It's the biggest ego trip of all time. The blackout was not motivated by character, or story, or anything - it was David Chase's ego (coupled by, ironically enough, a lack of balls).
Some suggest that the blackout is actually Tony getting whacked - and, if one chooses, one can recall the scene with Tony and Bobby in the first episode of the 2nd half, sitting in the boat, talking about how when you get hit, you'll never hear it. Clearly, the guy going into the bathroom is a reference to Al Pacino in the Godfather - it couldn't have been more obvious. (Hell, mgrasso sent me this link which details how there were multiple people in the diner who had reason to shoot.) But I don't buy it. Phil was killed; Phil's own crew practically endorsed the hit; and if someone on Tony's side had flipped over to the government's case, why on earth would they whack him before he got brought up?
Some also suggest that the blackout simply meant to reinforce that the sense of dread and foreboding that we were feeling as we watched is what life for the Soprano family will be like for the rest of their lives; paranoid, afraid, meaningless. (Also, Meadow would eventually have to learn how to parallel park.) Except, why the blackout? That same feeling would have been there if there was a fade out.
Instead, what actually happened was that David Chase refused to decide how to treat his characters and his story and left that decision to us. Normally, that would be admirable; except that, here, it was the very first time we were offered that choice. It's cowardly and dishonest. And by making America think that our cable had been blown out, it actually COMPLETELY shattered our willing suspension of disbelief.