My N800 has a touchscreen issue too. I bought it in february, and that's long after the fixes mentioned in this post.
The right side slowly becomes less sensitive as the device warms up. Not to the point that it won't work at all, but considerable nevertheless.
I won't bring it in for repair as I don't trust Nokia support with it.

The touchscreen of the N800 in general is not as good as touchscreens on other portable devices like PocketPC's. My HP iPaq has a WAY beter screen that is much more responsive. It's a treat after using the N800.

N800: click, -wait-, action
PocketPC: click, action!

A shame really.