Voted 60-70%. I think most of the work on cars and tracks is pretty much done, and right now they will just be finishing the codes on those.
Online and game mechanics are probably a tad behind (wheather and damage, things like that).
Call me highly optimistic, but i've read some rumblings of GT5 having 1000+ cars. I can't get a link to post here, but i've read that more than once, that the goal is 1000.
But rumors are rumors, i guess 500 is a reasonable number to work with. With time i trully believe that GT5 will exceed 1000 cars, but that will happen because we will have a lot of downloadable content, kinda like rock band.
That way the game can be done for a (hopefully) worldwide release in x-mas of 2009.
Like most of you guys, i wish i knew something...