This is one of those "it's so obvious I can't believe I've been using OS X for two years without figuring it out" hints.
It's always frustrated me that the Finder wouldn't show previews of XML, CSS, XSL, or any other text files without a ".txt" suffix. I always assumed there was a table someplace that mapped file extensions to a "preview as text" property, but I had no idea where the table was.
I still don't know whether that table exists or how to modify it, but I've found another method that works: simply set the Finder info type field to TEXT. A simple
find . -name *.xml -exec SetFile -t TEXT {} ;
is all it takes for XML files (within the current directory tree) to be previewable, assuming the Developer Tools are installed (SetFile is included with the Dev Tools).
D'oh!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030510093445143