More info about XMP tags in Vista can be found here:
http://blogs.msdn.com/pix/archive/2006/08/16/702780.aspx
There is a toolkit here that does XMP, but it looks like it's not being maintained any more. http://www.ozhiker.com/electronics/pjmt/
There's also some sample code here that looks useful: http://photography-on-the.net/ee/beta/cs_xmp_to_exif.php
Bottom-line, from looking at both of these code samples, is that you just open the file and read in the XML data using standard PHP commands. Then you can either manually parse the data, or use PHP's XML library. If I open the JPG file in notepad, I can see the XML data there in plain text.
I've finally added a sample file to the bug this morning as well.