I open mine all the time! When I get around to it I'll make a teardown or something.
Anyway, when you open the battery cover, there are four phillips screws, relatively small. I think I used PH0 screws. Then the bottom ones (close to the MicroSD) are torx, I believe they are T5? Anyway unscrew all of that, pull the display out, and the keyboard should just pop out.
Happy to see another Nokia N900 user!