As much as the lead up to the "black out" suggested that Tony should have some fear, then we as the audience must assume that such fear is warranted. Now, I cannot say if any of those suspicious people in the restaurant were there to kill him, but it is not out of the realm of possibility. Neither, however, is the chance that Tony was able to finish his meal with his family unmolested, at least for this evening.
I have to say this strikes me as the most meaningful ending - that Tony Soprano will go on for the rest of his days as jumpy as he was that last time we saw him. I think this is what Chase wished us to feel and he succeeded brilliantly in not just this episode, but the previous two as well. We were witness, for a moment, to know what it must be like for Tony every second of the day. If Tony "gets" anything, he most likely gets that same feeling Carmela discussed with him - that he walks around every day not knowing that a piano is dangling just over his head. Now, I think, he knows it's there. He just doesn't know when it will drop.
And, for the sake of argument, let's assume that there were killers there. The Tony Soprano Chase built for us would have somehow escaped from that as he always has. If anything, Tony is a lucky man (and apparently even more so after Chris died, he finally admits to Paulie.)
Either way, Tony gets up from that table and moves on in life - always with the knowledge that someone is out there waiting to gun him down, that an indictment may rain down on him at any moment, that his children will not turn out like he desired and that his wife will never leave him, but will never be truly happy with him either. Oh - and his shrink won't even call him back. Sucks being Tony Soprano.