The usual way to do rotation to portrait mode in Maemo 5 is to set special flags on a window and let the window manager rotate our window and relayout after the switch. It's not as fast as it could be, and task switching (even between two portrait mode apps) is only possible by switching to landscape mode. Apart from that, things like the system menu don't work in portrait mode.
For some applications, an alternative could be to stay in landscape mode and simply rotate widgets without the window manager even knowing...
It's probably best described in a short demo video of how a simple media player could look like (sorry for the sloppy video, forgot my digital camera, so the webcam had to act as a replacement) - have a look, and tell me what you think. (For comparison, have a look at the older Panucci rotation video that uses the "normal" way of going into and leaving portrait mode.)