Qt is an application development framework. It's something applications use. To avoid confusion, this will only affect applications that are Qt based, and even there, only those that were explicitly written to take advantage of this tech. Just having a library in there won't automatically make everything pretty. As the N900 ships with no Qt based GUI applications, this means very little change in the 'default' applications, more the ability to backport stuff from Maemo 6 or the desktop, or simplify writing apps that run on both Maemo 5 and 6.