There is a BMW employee, Scott27, who is a member here and other various BMW forums. One of his duties is to drop bit and pieces of information regarding future models in these forums to seed and cultivate interest. He's definitely for real - most everything he discloses takes place.

Here's what he said about the biturbo X3 back in August '06:
http://www.xoutpost.com/183922-post15.html

My guess is that we'll see an X5 3.5si towards the end of the year.