A while back I was messing around with Yahoo Widgets. I thought it might be helpful to have a schema so that my XML editor would help with auto-complete and validation. As far as I could tell, there wasn't an XSD for the widget format.
So, I wrote my own. This one isn't complete, but it's a good start for someone who is looking for a starting point.
View it at http://monoport.com/1556, an ad-free pastebin site.