EDIT 11/5/12 - OFFICIAL TSB FROM SUBARU! YAY!!!!
This seems to be a common subaru problem so I took the seat apart and what did I find...
After unzipping the seats, there is a visible spring that prevents the seat back cushion from falling through the seat frame. This spring is attached to the frame by 6 hooks ( 2 at the bottom, 2 at the middle and 2 large ones at the top near the head rest area. It is also secured by 4 O-rings(2 attached to seat cover and 2 attached to wiring inside seat back foam)
The squeak comes from the top 2 hooks that attach the spring to the frame, the subaru insulation (paper LOL) in this area comes off and rubs metal on metal inside the hook loops in the frame. This causes squeak, and it fix it just pull out the spring and insulate the 2 hooks at the top of it with tape or cloth or something. I used duct tape and it worked fine. See below for a diagram i drew up.
Here is the bushing part label!