Here's what I wrote that in forum, in case you missed it (that link to it above doesn't seem to work):
Under the current implementation of Quartz Extreme, the actual drawing of things to a window, such as text, graphics, UI widgets, etc. is not accelerated. What *is* accelerated is the the compositing of windows onto the screen in layers, after their contents have been rendered by Quartz (and thus the CPU did the work, not your graphics card).
Ideally, you'd want everything to be rendered by the graphics card, but unfortunately, that isn't entirely practical. Things like the special algorithms Quartz uses for font smoothing and vector graphics really can't be accelerated well, even with more modern programmable video cards. It would take a very custom graphics card to be able to offload *all* of Quartz onto it.
You also don't really want to be doing this piecemeal; if you can't accelerate all of Quartz via a video card, you'll have to shuttle fairly large chunks of data to/from the video card so that the CPU can render what little bits of it (say, some text) that the video card can't handle. This takes time and bus bandwidth.
In an ideal world, I suppose we'd have a super-customizable video card onto which Quartz can be entirely "uploaded" to the video card and executed from there. All that would be sent to the video card would be simple Quartz drawing commands, and the video card would do all of the bit pushing and compositing right there, leaving the CPU free.
I have no idea if this will ever happen or not, and I'm sure I'm simplifying some of the technical issues -- but that's it in a nutshell. Accelerating Quartz isn't easy -- Quartz Extreme is one step in the direction of offloading the graphics rendering to the video card.
So what do you get? Well, once a window is already rendered (and remember, under OS X, menus, the dock, even icons on your desktop are all technically windows), it can be drawn in any position on the screen, at any rotation, at an scale, at any transparency, essentially for free. A bouncing dock icon, for instance, shouldn't chew up nearly as much CPU as it does now, and window transparencies and shadows should all be rendered "for free".
What don't you get? The actual drawning of the contents of a window (the text, graphics, interface widgets, etc.) are still drawn by the CPU.