I believe it is possible, to be honest. I saw a video shortly before the game came out that slowed down the part in the Oval Office, and there was a man with a leather jacket seen before the entire screen went white. It was really brief, and it was possible that it could have been Leon, but I don't think it had the stripes around the arm. In the end of RE5, it's pretty clear that he died, but this is Wesker we're talking about, and as a side note, nothing dies anymore. In short: It is possible, yet very unlikely as how many facts are opposed to it.I would like it to happen since I'd like to see Jake fight Wesker when Wesker tries to destroy the world again, but again, still really unlikely.
I personally have had this discussion with a few friends of mine and have a few facts to back up my claim. "We have not seen the last of Albert Wesker." In RE5, when Wesker is confronting Sir Spencer in his stereotypical RE fashioned mansion he tells Wesker that there were multiple Wesker children, but Albert was the only one to survive the injections. A awesome heart-ripping scene later, on the tanker before you reach the bridge you enter into the high-tech communications room, and found in a leather-bound briefcase tucked in the corner we find a file written for Spencer about Albert's escapades, and signed at the bottom is Alex Wesker. Before I hear doubt, in Spencer's journal he states there is a 12th candidate in the Wesker project that can mimic other's abilities just by observing them. Knowing this and the fact that Albert is Wesker child 13 I have come to the obvious conclusion that they are probably brothers, and Albert knowing his brothers ability, used him to fight Chris and Sheva in his place. On a final note, does anyone have a doubt that the most influential antagonist in Resident Evil history is actually dead. I rest my case.