The PR 1.1 firmware update brings portrait mode web browsing to the N900. In order to enable auto switching between landscape and portrait when the phone is turned hit Ctrl + Shift + O on the N900’s keyboard. The next time you tilt your N900 to portrait, the web browser will automatically switch as well. You can turn this behavior off by hitting Ctrl + Shift + O again.
I’ve put together a very quick demo of the portrait mode browsing on the N900, have a look.
This is a very welcome move, although personally I would have prefered to see the Conversations app get portrait mode support first. He’s hoping that will be done in the next firmware. The portrait mode is an undocumented feature and this means that Nokia is still working on all the nuances associated with portrait browsing.