For IM/SMS/EMAIL it is very simple, its just as i use my previous phone the E90
I get an incoming SMS i see it in portrait on the front screen, there i read it, then i decide ok i need to reply then i switch open the E90 and use the qwerty keyboard and the big internal screen for replying.

I would do the same for the N900, i read incoming stuff quickly in portrait mode, then when i reply i open the keyboard and switch to landscape.