A new application that could be included with Ubuntu 7.04 “Gutsy Gibbon” will make configuring dual monitors easy.
DisplayConfigGTK will work similarly to how screens are configured in Windows. Currently there is no way to easily extent the desktop to a second screen in GNOME. Xorg 7.3, when it is released, will allow reconfiguration of the screen on the fly. Unfortunately, only two monitors will be supported because of the complexity of dealing with more than two monitors at a time.