It's not so big because it just makes sense, and has been hinted at before. They will still be entirely different UIs, but it makes sense to share components and aim for a common development platform (to some extend).

It is peculiar though, that this is coming up when Maemo is just on the verge of switching to Qt. Maemo 5 and Moblin are both built around Gtk and Clutter at the moment.

I wouldn't be entirely surprised if Maemo 5 would develop a life of its own, and at least for some time would coexist with the Qt branch of Maemo 6.