Agreed, but not just that, anytime you're carrying a bag (shopping) or suitcase or laptop bag, or in my current situation, using a crutch after a sports injury. Holding onto a rail on a bus or train; browsing while eating pizza or fried chicken, and I do this a lot with my Blackberry at the moment. You can go on and on.

I would say email/SMS and browsing would be the main things I'd want in portrait mode. Perhaps the media player for music - after all most people keep their phones in their pocket in "portrait mode".