i managed to install vista on a partition on my main hard drive using boot camp which works as usual and then installed windows xp on an external usb drive using a modified windows xp disc (google for it in not allowed to post a link). the problem is that as soon as you install windows on the external after installing vista on the boot camp partition, you can only boot into windows xp using the boot selector. this is the problem im having except i removed vista and now i cant even boot into xp.
I can confirm that the directions do work (in a sand-boxed environment) on VPC 2007, so I'd imagine that with a large enough partition the same would work for Bootcamp. Just follow the directions to get XP installed on bootcamp, then follow the directions on the link above.
It takes a while to do normally so be patient. I spent about an hour and a half, but I was running the installations from ISO's.
Note: I used the DISKPART directions as opposed to the GPARTED ones. Just make sure that you read the help for DISKPART (type "help diskpart" without the quotes).