To give you an idea of how shallow (as opposed to "deep") the portrait support is for N900, you can't exit full screen in the browser when in portrait mode. Basically, Nokia just decided they didn't want to deal with having to adjust any UI elements for portrait mode, they just rotated the page rendering.

Try closing the browser in portrait mode, or selecting a new website from the address bar. Can't do it.

Nokia just says deal with it, they "don't have plans to support this."