In past Maemo releases, the on screen keyboards have been summoned by:

1. not having the hardware keyboard (if any) opened
2. not having a Bluetooth keyboard (if any) connected
3. tapping (or otherwise moving focus) into any text entry area on the screen

I've not yet used a Maemo 5 device, but I've heard nothing to indicate there's been any change in that behavior.

Well, the N900 is the first Maemo device with an accelerometer, so no, there's nothing from previous Maemo devices that transferred forward in that area. However, through a lot of trial-and-error work, and some time spent teasing information out of Nokia developers, the community has obtained and documented a solid understanding of how to add portrait support to applications.