From the Oregonian: An article about building the Sullivan's Gulch bike trail and proposed bike park at the Gateway Green near Rocky Butte.

http://www.oregonlive.com/environment/index.ssf/2008/10/hold_your_sprockets_new_bike_p.html

The first incarnation of the Gateway Green Vision Plan has been released by the Portland Development Commission.  Page 17 has a map that gives a good overview of the site plan..... and yes the plan allows for a large area dedicated to mountain biking.