With the slideout hardware keyboard, N900 is designed to be used in landscape mode only. The only time you can switch it to portrait mode is when you make/receive a phone call.
Only then you get the on-screen keypad to press the phone no you want to dial.

But, if you wish to choose a contact from your Contacts list to make a phone call, you have to open the slideout keyboard (which automatically rotates the screen to landscape mode), enter first few letters of the name, select the right person in the list presented and close the keyboard. The screen will then rotate back to portrait mode.

For photo and video, you can shoot it in either orientation and it will playback in the orientation you recorded it with, which I think is pretty cool, although I don't understand why would anyone want to record video in portrait mode when all the viewing platforms, such as TV/monitors are all in landscape mode.