If what FS said was true, then I think it symbolizes the fact that for Tony Soprano, EVERYONE is a potential enemy and he always has to keep his eye on who gets up from a table, or walks into a restaurant, because that person might blow his brains out. Maybe it WAS Phil's nephew or the two guys who shot him.. or maybe it was done to illustrate that point; to him, everyone looks like a possible threat.

In fact, I think the entire ending symbolizes that: Tony mentioned that when you die everything goes black. The abrupt ending was meant to show that Tony never WILL know when his end is coming, because it can come from pretty much any avenue. He has to be paranoid for the rest of his life, always looking over his shoulder. He can try to be a family man, a loving husband to his wife and a loving father to his children, but everywhere he goes, he has to be wondering if he's going to get whacked and never see it coming.