Rather than enticing existing Windows XP users to switch, Boot Camp will be primarily attractive to current OS X users that are lusting after certain Windows XP applications, such as games. This makes sense -- they're already accustomed to performing most tasks on OS X, and only need to switch over to Windows when they feel the urge to game.
Ultimately, with Boot Camp, Apple is only helping Microsoft sell more copies of Windows XP. How sweet of them.
I am a unique case in that I eventually want to buy a Mac Mini anyway, so Boot Camp just sweetens the deal. But I am atypical - I intend to wait for the Merom-based Minis and make it a silent-PC workstation for my scientific apps. And as a unix development machine. I might do some media streaming, but that would be secondary.
For most PC users, I don't think that the presence of boot camp will sell more high end machines, though it might definitely help lean some people inclined towards the lower end for experimentation. If anything, boot camp kind of erodes the Apple Brand, by conceding that all OS X users arent fully served by their OS. This undermines the (quite valid) argument that you can switch to a Mac world and be fully independent.
Look, for years Mac aficionados have argued that everyone shoudl switch to Mac and abandon Windows. Why now are those same partisans arguing that software that lets you run Windows instead of Mac OSX - on your Mac! - is going to facilitate that switch?
It all strikes me as a bit schizoid. But that's life in the Intel-Mac era, I guess.
UPDATE: I predict that in five years Apple will get the true Zen and abandon PC hardware entirely, and become a software and media company. They'll still make iPods as loss leaders to get you hooked on their apps. They'll buy Yahoo and go full-on folksonomy for personal search, web search, and tie it all into content and ads. The majority of their revenue will be selling music tracks and video tracks (including the complete Disney and Pixar archives) via the iTMS, which will be completely hardware-independent (iPod, personal PC, in-dash on your car via 802.11n) and their largest single expense will be annual licensing fees to Apple Ltd (ie, the Beatles) whose trademark infringement fees they will just have absorb as an operating cost. McCartney will sit next to Jobs on the board.
mark my words!