Use Windows XP Upgrade Version With Boot Camp
Apple says in their installation guide[pdf] that a full-install Windows XP disc, no upgrade versions, is required for use with Boot Camp. Not entirely true. I was able to use Windows XP Home Edition with Service Pack 2 along with a Windows 98 install disc and an external USB CD-ROM drive.
I followed Apple's instructions as usual, except for the substitution of Windows XP upgrade version. Then when the Windows XP upgrade installer prompted me, I inserted the Windows 98 disc into the external CD-ROM drive. That's the only difference; installation continued as usual.
Note: You can't boot the Windows XP disc from the external CD-ROM drive if it's USB; boot the Windows XP install disc from the internal optical drive. I also actually used an USB to IDE adapter to connect an old CD-ROM drive I had laying around. I found some help at the Windows on the Mac forum at Mac Rumors.