Like X_Jasper said, e-mail the main website to see if it is. If it's not the real deal, then we can just simply forget about this video and move it to the "humor" section instead.

EDIT:
I kept looking all over the net to see if the original website [project magenta or others] have the video posted. None of them do. It's very strange that this link has PM's web address listed but isn't showing up when veiwing from their main site. Strike me as odd. Me thinks it's a hoax. If it is, then it's a dang good hoax.
However, I am noticing some visual effects that indicate that is is FSX. Indications are:
1. Bump mapping [notice the tarmac and runway]
2. Self-shadow effects [notice the airport terminals early in the video]
3. A little bit of sun shining on the nose of the 737 [during the close-up shot]
So maybe it is. But don't forget that most of the "stunning" visual effects are optimized for DX10 and Vista [which won't exist until next year]. So don't expect to get the whole cake when FSX comes out.