I’ve been to a few SilverLight events and read about it on the web, I’ve even played with it a little, and I think it’s very interesting. But, one thing I’ve learned from all of these experiences, beside the fact that the average dot.net developer feels awed when he sees how to create a rectangle with a gradient fill, is that Microsoft is pumping the fallacious fact that SilverLight is SEO (Search Engine Optimized) because it uses external XAML files, which are basically plain XML files. In the last event I were at, the presenter repeated this "fact" with such determination, which made me jump out of my seat with rage, well not really rage :), I’ve just explained it to him nicely why it’s not true. He was more modest with he’s determinations, afterwards.
Even though it’s a known fact that SilverLight isn’t just SEO out of the box, I still see this being repeated all over the web. You should question authority, and shouldn’t believe everything you’re being told, even if it’s Microsoft.
Currently, search engines don’t even bother looking at XAML files, IMHO they won’t start parsing it any time soon. The same way google don’t parse dynamically loaded XML files, since it can’t do much with it, you can’t get much out of a parsed XAML unless your looking for a Rectangle that is positioned at x=0.1232 and y=33.4355.