<p>I'm not committed to any particular GUI tookit or anything - just needs to be Java based. I want to do simple syntax highlighting ( XML and XQuery ) inside editable text areas.</p>

<p>My only candidate so far is Swing's JTextPane, as it supports seems to support the styling of text, but I have no idea how to implement it in this context.</p>

<p>If a particular toolkit has something like this out of the box, that would be awesome, but I'm open to doing this by hand if need be.</p>

JSyntaxPane handles XML and can be extended
http://code.google.com/p/jsyntaxpane/wiki/Using
Or, it should be possible to extract the NetBeans editor, but that would probably be more work...
[edit] btw, I got the XML info from
here
...  it doesn't seem to mention it on the google code pages...