I took apart my n900 last night to swap the screen and try to fix this.

Turned out there was something broken in there. There is a little piece of metal, it's curved with a piece of plastic on it. It was 100% loose and I couldn't figure out where it went. My plan was just to take it out, and then maybe wrap the stylus with some tape to keep it in place.

Turns out the stylus still locks in place even without it. I can feel a lot less resistance than before, but still locks in place.

Swapping the screen was pretty hard, but if you just need to reach the bottom of the screen it's not hard at all.