An official version of the XapOptimizer is now released by ComponentOne:
In one click you can reduce the size of your Silverlight apps up to 70%
and secure your code with obfuscation. Add ComponentOne XapOptimizer to
your dev cycle with build automations, backups, and limitless options.
On the ComponentOne's official site you can find plenty of additional info about the product's features, learning resources and testimonials. Also you are able to view a demo and try it for free.