WPA
The truly frustrating thing is, once again, Windows Product Activation, aka "Bill owns your soul, so bend over and take it like a man!"
Ultimately, Redmond can just switch off their XP WPA service, so no new XP licences can be installed, regardless of what Dell or anyone else wants. They can make WPA work only for Vista, or XP only on SCCs, or whatever the hell they want.
You'll be forced to use whatever they say you must use.
Sure, one can argue that they'd never do that, the backlash would be too great. But they still have that ability, which doesn't make me sleep well at night.
Oh wait a second: If I really want to play games (Windows' only attraction) I can get a PS3 or a Wii. And I can do actual computing on Linux. At work I'll have to use Vista, but it'll be the company's problem to pay for and support it.
Windows' share of the home market may well start shrinking much more quickly than their corporate market-share. Home users now have more choice, and aren't as beholden to corporate policies or compatibility with corporate or specialist software.