I visited Nokia flagship store in Helsinki today to check this on the prototype N900. Both phone application (dialer) and contact list can be used in both portrait and landscape, with auto rotation unless the keyboard is drawn out. Perhaps you meant a hypothetical case when the dialer is fixed in portrait by users choice -- it is possible too.

I was a bit suprised, however, that the simple looking
sms, and on-screen keyboard were only landscape. Did not try calculator or notifications.

The firmware was old 34-8 if I remember correctly.

I am happy -- my use cases for single handed portrait would be checking the just arrived sms and answering/placing a call. Only rarely also writing a sms on an on-screen keyboard -- the fixed keyboard is way better. As for media player, for controlling music single-handedly I would mostly use remote control buttons in some headset, not sure if it is possible here. Browser portrait mode may be coming too (see Michele here).