I've read the tehkseven page, and I'm sorry to say but Nokia isn't using cheap screws. The problem is analogous to PEBKAC (problem exists between keyboard and chair), that is the person holding the screw driver is a butterfingers.

I've taken my n900 apart 4 times and all the screws are fine. It's simly a matter of cross threading and overtorquing. Either one of those things will snap those tiny screws.

As to the replacements, there is a thread that lists sites that sell n900 parts.