So I was looking at XMLBoard a couple days ago, and since I'd taken an interest in XML earlier, I pondered if a PHP forum based on XMLBoard's DTD would be worthwhile. Sharing the DTD wouldn't really gain anything beyond the ability to migrate a forum from one platform to another. Sharing a forum between two otherwise unrelated sites would be an interesting option, though.
XML for a forum certainly has advantages. The first obvious one that beats SQL hands down is that the relationship between messages, regardless of how deep a thread goes, are represented automatically. I like that. Another good benefit is the ease of searching. There are, of course, downsides. Random access isn't the easiest thing to do. And how do you lock the file when writing posts? Well, there's a small project to look into some time.