I'm back to comment on my experiences with the 30-inch ACD and the 2690.
I found that Apple has refurbished ACDs at a very nice discount, so I decided to take a chance on one. I found that the display behaved pretty much exactly like my 20-inch ACD, but with one big difference: because it is so large, the angle between my eyes and the corners of the screen were large enough that particularly in the lower corners, image quality fell of quite substantially. It wasn't noticeable with bright material, but for dark material it looked like there were spotlights on the corners. The other problem was that it was just too big. The lack of vertical position adjustment put the menu bar too high.
So the ACD went back and was replaced with a refurbished 2690. Image quality is excellent with no apparent off-angle problems. As well this turns out to be the perfect size display for me. With larger text it can sit a bit further back and still be easily read by my tired old eyes, and it can be adjusted so that the menu bar is not too high. Esthetically it's a big step backwards have a big piece of black plastic replacing the elegant aluminum ACD, but I'll have to get over that.