Hi Virgil,

I've replied this in my blog, but thought it'll be helpful for other developers here as well.

Regarding the threading, unfortunately, Silverlight doesn’t ship with true multi-threading architecture in the latest v4 release. So all the MDI windows will share the same thread with the root application.

However, the good news is that since Silverlight is built on the WPF-style rendering architecture, most asynchronous callbacks or animations are still smooth even if it’s running in several windows together.

There are several possible techniques to simulate multi-threading for a specific UXWindow (let’s say if you need to execute a long process), you can do that by using DispatcherTimer or BackgroundWorker.

Hope this helps,
Jimmy.