"Ease of use, and effective integration, are key values in our user experience. We care that the applications we choose are harmonious with one another and the system as a whole. Historically, that has meant that we’ve given very strong preference to applications written using Gtk, because a certain amount of harmony comes by default from the use of the same developer toolkit. That said, with OpenOffice and Firefox having been there from the start, Gtk is clearly not an absolute requirement. What I’m arguing now is that it’s the values which are important, and the toolkit is only a means to that end. We should evaluate apps on the basis of how well they meet the requirement, not prejudice them on the basis of technical choices made by the developer."
In evaluating applications for the default Ubuntu installation, Canonical looks at whether the application in question is free software, best-in-class, integrate well with he rest of the system, is accessible to many, and provides a consistent look and feel.
In order to ensure that plays along with the rest of the Ubuntu stack and its largely GTK application base up to now, Canonical is driving the development of dconf bindings for Qt (by means of contracting one of the developers) so that Q applications have access to the same settings framework as the rest of Ubuntu.
In Mark's blog post he goes on to talk about the relationship between KDE and Qt and that their choice of throwing greater weight behind Qt isn't meant to criticize GNOME/GTK. Though with Canonical developing the Unity Desktop for Ubuntu to replace the GNOME Shell by default, they are clearly looking to become less dependent upon GNOME.
This news from Mark also just comes days after it was found out they are developing a Unity 2D desktop and that's using Qt.