You guys can try to play with the 2 t6 screws that hold the frame in place.

When I disassembled my n900 and put it back together I noticed that hitting the space bar felt different. It was the little strip from the frame that was actually moving up and down.

I backed the screws off and then tightened them just until I felt them bottom out, not even the tiniest bit of pressure after. Much better now.

I would guess that is the only difference between all these n900's. How tight the screws are. Too tight and that strip will bow more.