@reinob:
Here's a N900 disasembly video:
https://www.youtube.com/watch?v=aVEHm325-z8

The keymat is removed at 3:50, while the display unit is already removed.
I can't find it but iirc there was a video some years ago showing some guy casually replacing an N900 keymat in like 30 seconds, without any sort of disassembly except for the keymat bezel.