Not sure there is much of an overarching plan, to be honest, but I can't speak for ArenMook.
From my perspective and best guesses, NGUI will continue like it is with updates as needed, as long as it makes sense. It will be interesting to see how the uGUI will affect NGUI when it comes out, but it's impossible to say right now.
It will be interesting to see if all these alternative UI solutions (NGUI, DF-GUI, NoesisGUI, CoherentUI etc.) will be obsoleted entirely or they will offer something the built in solution doesn't and thus creates their own niche like NGUI did back when it started. uGUI will be serious competition of course, because it's built in and free, no less. So I imagine most people using Unity Free will roll with that, and maybe first timers too, just for lack of trying the alternatives. It's only when the 3rd party tools offer something distinct that they will be favored over the builtin.
NGUI will likely be hardest hit of the UI alternatives, both because it's the most used one and because the features overlap directly more than any of the others. All the users who already use NGUI will likely have some inertia moving to uGUI if they want to, because shifting middleware mid-way in development is generally a bad idea (hello, duke nukem forever) and because programmers are a stubborn bunch.
Let's have Unity 4.6 out first and the actual uGUI up and running before making proper comparisons, because all we've seen is a newly released video, not actual working projects - yet.
