ARG! Seems like Quartz 2D Extreme does NOT work on all Quartz Extreme-capable graphics cards after all. On my Mac mini, the menu item is dimmed and says "Quartz 2D Extreme not supported". So, what ARE the requirements for this? Same as Core Image? I STILL can't find any explicit mention of it on Apple's site, although the Quartz Extreme page sure makes it seem like 2D is included. Their graph shows Quartz Extreme under ALL graphics work. Is that just a lie?
By the way, the resolution switching thing does work, but it's very much not ready for prime time. Maybe it would look better with a better graphics card, but on my Mac mini, it looks terrible. Scaling things up looks decent (hardly perfect), but scaling things down looks terrible. Either way, there are a lot of strange artifacts, like grid lines in the menu bar (???). Also, some things are totally out of whack, like Safari's Bookmark bar, which seems to go further and further underneath the navigation bar, to the point where it's completely invisible at the minimum setting. Weird.
Of course, that's why Apple didn't enable it.
It definitely looks promising, but it's got a ways to go before it's consumer-ready.
Edit: Actually, after a bit more testing, I realize that the weird gridlines don't appear when you use round multipliers 2x, 3x, 1/2, etc. At 2x, it really does work very well. 1/2 still needs work, though.