I too have had this question. In my research, I have come accross pages that indicate XML support is currently under development. There is a patch for PostgreSQL version 8.2.1 found here : http://chernowiki.ru/94.html
Version 8.3 of PostgreSQL already has this functionality. I have only just downloaded this patch myself.
I am not able, at this time, to give a full answer. I can only redirect you to this link :
http://wiki.postgresql.org/wiki/XML_Support
If someone has already figured out a full answer, feel free to contribute.
( Be advised, the links given above are valid at the time of this writing. Your milage my vary. )