A new Ubuntuone-Installer version was uploaded to the Ubuntu 12.04 Precise Pangolin repositories today and with it, Ubuntu One has switched to a new Qt interface by default, replacing the old GTK version.
According to a bug report, the Ubuntu One team is no longer supporting the GTK interface so it will be removed soon.
Among the reasons behind this decision are: deprecated pygtk bindings, uniform interface across all the platforms, usability and others.
And the Qt UbuntuOne Control Panel interface:
Currently, there's a bug and launching UbuntuOne from the Unity Launcher, the installer fails. This can be solved by manually installing the Ubuntu One Control Panel Qt interface:
sudo apt-get install ubuntuone-control-panel-qt
The latest ubuntuone-installer package also installs the new Rhythmbox UbuntuOne Music Store plugin, replacing the Banshee one: