I'll certainly try that in my software. So the way to do this seems to add my own button to the GUI of my app (using the appropriate Nokia system icon) and then call the D-Bus session bus interface com.nokia.hildon_desktop using the signal name: exit_app_view (according to a post on the developer mailing list by Nokia's Kimmo Hämäläinen).

PS: He also mentioned "We will also have a HW key shortcut (ctrl+backspace) for that". Is that implemented in the N900?