Well somewhat quickly.
Like everything else with X it's all compomentalized and they aren't going to break backward compatability. They'll keep the old framework, with some clean up, making things more modular (seperating Xlibs from the X server and such). But the movement is towards OpenGL, no doubt about it.
With the next version of GTK 2.x (2.8, I beleive) they will have Cairo with Glitz as a optional rendering method (as opposed to Xrender as the default). That wil take care of the OpenGL accelerating of the window, widgets, and text. (also make things look much nicer, especially on hi-res displays), if you want it. Cairo is interesting because it can take multiple rendering methods, like traditional X render, Framebuffer, or Glitz (and thru that OpenGL and thru that hardware acceleration), or be rendered on whatever system is aviable to Windows and OS X users. Then it can be rendered to a veriaty of outputs like the display or to PNG files and even to PDF files (which would make printing in Gnome MUCH MUCH simplier)
Qt will do something similar..
Then you have experimental Window managers now with OpenGL acceleration.. most notiably Luminocity, which is a test bed for Metacity, which is the default window manager for gnome.
Then you have the actual X server itself, a experimental version of the X.org window manager is XGL, which has OpenGL acceleration for windows and composition toys. For instance waima author created a some little toys like moving windows drop shadows (follows the mouse or something like that), and they recreated the OS X expose' with a couple hundred lines of code. This stuff, of course, is just to show off and is modified version of xcompmgr.. but eventually that sort of functionality will be implimented in the Window manager itself.
So OpenGL-accelerated Cairo with GTK or QT + OpenGL accelerated Windows manager + OpenGL accelerated X window server = OpenGL driven free software desktop.
Of course they are keeping it all compomentalized in the traditional *nix fasion.. you can run glitz with a normal window manager and normal X server or you can run XGL with a normal gtk widgets and a normal window manager or you can run a OpenGL accelerated Window mananger with normal widgets and normal X server.. or any combination that would be practical. Also they can do this without breaking backward compatability (baring the bugs that come from new developement, of course).
Also having everything OpenGL will make making stable drivers much easier. No more of the dual-nature of the X server drivers. Just one driver... And for systems that don't have decent drivers then it will simply use Mesa software acceleration. And because DRI drivers, and most others, are extensions of the free software Mesa libraries, cards that are only able to accelerate some things, but not others, will still have full compatability since what they cannot accomplish with hardware will be substituted with software.