Corso85, the reason why the N900 and Maemo 5 is predominantly in landscape mode is due to its heritage from the previous Maemo systems (which were all landscape). I also believe the GUI of the OS and many of its default apps are created using GTK. So it requires more time and effort to create a portrait mode. Hence the reason why most if not all apps and built-in apps are landscape only. Combined with the hardware keyboard (and again it's history as a Maemo tablet) landscape was chosen as the predominant mode.

Meego seems like it will be built predominantly in QT which has much better support for portrait mode. Hence it's better for switching being portrait and landscape.

To sum it up, will you see a virtual keyboard from Nokia in Maemo 5, and portrait mode across built in apps (3rd party apps do support portrait mode, especially the ones using QT since there's better support for them). Probably not (especially not from Nokia). If you do see it, it's likely going be the result of a community provided xrandr hack (which won't be pretty, parts of the screen will be cut off and inaccessible) and a keyboard app. Which I believe the keyboard app is being made.

This is how portrait mode was accomplished in the n800/n810. Though it wasn't pretty.