Carlos Garnacho has been working on adding X Input 2.0 support to the GDK by adding in a GtkDeviceGroup multidevice-event signal. The GtkDeviceGroup is a container for grouping GdkDevices, hence its name. There has also been a few API changes to GTK+ and other work to incorporate X Input 2.0 / MPX. There is also a fair amount of work left, which is detailed on the GTK+ MPX Wiki.
More information on the work done by Carlos can be found on his blog. There is a GTK+ MPX Git repository for those interested in trying out a version of GTK+ with multiple pointers before it enters the mainline GTK+ code-base.