I have 1680x1050 on my 15", and IMHO that's the upper limit... and I'm notorious for preferring higher resolutions than most people can stomach. Out of 100 people who ever looked at my screen, 99 have said "woah, that's just microscopic, how the hell can you work with that?"
1920x1200 on 15" is a niche product... only a small fraction of Dell's customers order their 15" laptops with 1920x1200 (they've offered that option on their professional laptops for many years). 1680x1050 isn't that popular either... 1440x900 is the bestseller. Given that Apple's product line is very limited and has rather few BTO options, I doubt they would introduce something as far out as a 1920x1200 option, when they don't even offer antiglare on the 15" yet...
On the other hand, 1920x1200 might be easier to live with on Mac because a lot of stuff is bigger than in Windows. OS X uses larger text for menus, dialogs etc, and the dock icons are huge by default. But then there's a lot of bitmap graphics in applications... I use this music app called Reason and some buttons are as small as 8x8 pixels... that's, what, 1.5x1.5 millimeters on a 15" screen with 1920 horizontal? Yikes.