An XMLWiki is a concept I've been toying with for a while now, and much more so since I've seen SnipSnap and started tiptoeing into Cocoon.
Besides being (obviously) an XML-based WikiWikiWeb, XMLWiki would leverage Cocoon as the excellent page structuring/rendering pipeline it is in order to seamlessly blend together all sorts of (relatively) structured data.
(Yeah, I know, I sound like a Marketing guy. It comes and goes.)
The idea is very appealing, especially where it regards referencing (XPath springs to mind) and exchanging data with other information stores.
However, more practical concerns need to be addressed at present, and so I am focusing on something called LiveWiki. Unless the name is taken, of course. :)