I have a question:

When a software goes to vertical mode, is this his window that rotates 90deg and resized or is it the X Window server?

If it's X Windows, it's no need to rotate 90deg widgets: simply reverse their coordinated X and Y.

And during the first rotation, after placing a Widget, Maemo can correct automatically coordinated of the Widget if it exceeds the screen.

After, be it Maemo which corrects any single the position of the Widget, or be the user moves it, Maemo remembers hes position in horizontal mode and vertical mode.
Thus, the widgets will have different positions for the vertical mode and horizontal mode. Like the very good idea of Elimoon8.