Some aspects of XP don't seem to fit completely into the world of system administration. The most difficult seems to be pair programming. While pair programming might not be a viable option for most sys admins, the ideas underlying it fit very well. Another pair of knowledgeable eyes to look over plans, configuration files, scripts, and other products of our craft can be a boon. (I've had more than a few brainos caught by coworkers.)
I think wholesale adoption of XP into system administration probably isn't the way to go. Reading up on the methodology and stealing the concepts that work seems a much better way to add XP to your daily regimen.
Anyone want to pair up to go clean up sendmail.cf?