I figured it out. you have to set the user agent to something it likes, so adding the line:

before you actually request the XML file will trick the armory system into believing you are using one of the more advanced, modern browsers (in this case, Mozilla FireFox). Then you will get XML rather than the XHTML Transitional code from their server.

Case in point, my sidebar on my personal website (http://darkhelm.org) has a section listing my WoW characters, done through PHP, using the data straight from the armory.