jarpy has an interesting post up on installing Windows XP to an external USB hard drive without using Boot Camp to repartition his internal drive. This method makes use of the Open Source rEFIt utility which we've mentioned before. The process itself looks somewhat risky (e.g. jarpy disconnected his internal drive to avoid corrupting his OS X install) and playing around with third-party boot loaders is always potentially dangerous in my opinion. However, the payoff of a USB-bootable Windows drive is rather appealing, so it might be worth a look for some of you. Proceed at your own risk. If any of you decide to get this working, please share your experiences in the comments.