With regards to putting the faster NVidia chip in it, I don't know why Apple decided to put the slower chip in, i'm not on the design team. But according to NVidia's specs it pulls less power, so i'd bet thats why. the faster chip can turn its 3d unit off when it's not using it to save power, but thats pretty useless if this is supposed to be a CAD workstation replacement. If apple were to use it that wouldn't be useful at all as with quartz extreme running all the time you'd never be able to turn the 3d unit off.
With regards to the problem with viewing displays that have that high of resolution, the UI is going to start holding people back. I can zoom in as far as I want on my illustrator, indesign or word document, getting the image that big isn't a problem. The problem is what happens when you are browsing the web? sure you can make the text bigger, but then the images are out of proportion. And what happens to (like others have said) the buttons and scroll bars. Since monitor dpi hasn't really changed all that much those issues havent really been a problem. Users have just accepted the fact that things look a little bigger or smaller on different machines. However, if those are half/quarter sized, thats a little difficult.
People have been talking about resolution independent UIs for a while. I say BS, it would be too much of a pain for a bunch of reasons. What will probably happen, imho, is resolution of mainstream displays will increase rapidly over the next few years, and at some point we'll hit an upper limit, where people can't really see any more (probably around 220-270 ppi, IMHO). Then OS developers will standardize around it like they did around 72-90 ppi 15 years ago or so, and redesign their UIs around that. Until then you'll just have to squint or turn on the zoom feature in Jaguar. Whatever happens, believe me, this is something people have been thinking about for more than a few years.
ps: I saw a tiny tiny tiny little sony pocket-laptop deal at the Metreon in SF (which sucks btw unless you like little Gundum figurines and playstation) it had 1600xsomething-less-than-1000 resolution on a dinky little (less than 8 inches maybe) super-wide screen; looked like one of those little HP super-subnotebooks. wish I'd written down the specs. Probably makes this dell look like its running 320x240.
pps: sorry for such a long post.