This is really not a huge deal, but I was surprised to see that this latest drop still uses the “Definition” namespace URI. I know there was some discussion about this even at the PDC when Avalon was unveiled, being that it's not exactly the greatest URI on the face of the Earth.
I realize we're not in beta yet and of course nothing's set in stone, so I figured I'd raise the flag again so that it doesn't accidentally fall by the wayside without at least some discussion around it. The simplest thing would just be to tag it right onto the main XAML URI and end up with: http://schemas.microsoft.com/2003/xaml/definition.