I wonder if the look implies that Desmond has been to the future and knows that Sayid is now allied with Smokey? I had a hunch that Faraday himself might have been unstuck in time at some point prior to his death (else why the 'Desmond is my constant' in his journal?) and it's possible that during his mental illness phase prior to his dad recruiting him for the final, fatal mission to the island, he didn't jump to the far future, meet everybody during the endgame, cross paths with Desmond again in the near-past and fill him in a bit on what's going on, then jump back and promptly forget everything he had learned in the future? Might allow Desmond to know that Sayid is a baddie and may give him insight into what to do going toward the end of the series. Probably too much time-travel confusion for the average viewer though...