I've unintentionally gotten the UI into portrait mode this way:

make a call in Portrait mode, when the call ends don't close the phone app, just click the minimize icon in the top left-voila!, portrait mode on the desktop.

Problem is, the UI is stuck this way, and it's not pretty. To exit, press the power key, select "lock screen and keys", then press the power key again, and swipe to unlock while holding the N900 horizontally.