I've been looking for some answers regarding the macBook and apple's 30" display - and this weekend I got 'em, so I thought I'd share:
MacBook (15") running OSX, plus 30" display: Beautiful (this was expected). It'll mirror or extend the desktop. When setting the resolution, there are independed pop-up boxes to control the resolution on each screen. Very nice. (this was never a question)
MacBook running XP (via bootcamp), plus 30" display: I wasn't sure if this would work out of the box, but it does. It will only extend the desktop - won't mirror (maybe it can be done, it just blanked out when I tried). But when extended it uses the full resolution of the 30". I was pleased to see this (half expected XP to not be able to drive the 30" properly.
Switching between the OS's wasn't a problem with regard to the external screen.
Also, for what it's worth I hooked up an older (DVI, but not dual DVI) 12" powerbook to the 30" monitor, and while it can't take advantage of the full resolution, it does push the resolution nice and high - and the display puts some black bars on either side so it's not stretched out. Really, quite nice.
My requirements for picking up one of those displays (I know I'm lucky to be considering it) was that it run at full res with XP on the macbook, and that it's at least usable with my current 12" PB. Both worked nicely.
(For what it's worth, I most of this at the CompUSA in Oakland where they've got a macbook running XP and the guy was happy to move some monitors around.)