Maemo 5 is available in landscape mode mostly. Only the phone app, photo browser and some 3rd party application run in portrait mode. Thus, two hands are required that is sometime annoying. Now, Guyver from talk.maemo.org managed to run the Nokia N900 in portrait mode:
Hi, i somehow managed to get my n900 in portrait mode from a glitch. How weird.. lol
I probably cant recreate it again, but i just tilted the phone to launch the phone app then slide up the screen and closed the phone app.
Let’s hope that Nokia will add this feature in a upcoming firmware.