I'm simply wondering if it is possible to open one or more non-modal popups or dialogs, which are NOT limited to be displayed and moved within the bounds of the parent Silverlight control. Doesn't seem to be possible, even for out-of-browser installs. I hope I'm wrong :)
No this is not possible, Silverlight strictly renders to a single window (or if in Windowless mode the specific rectangle provided to it by its host).
As Anthony said this is not possible. I've heard from some of the people involved with Silverlight development that this is an often-requested feature and will most likely be added in an upcoming version. But I'm guessing it would only be available in "out of browser" mode.
While the previous answers are true, strictly speaking it is possible in browser mode to have a real popup. At the moment it can only be done indirectly and with a lot more work than a normal Silverlight popup Window.
Basically you popup a new Silverlight app (or second instance of the current app) in a browser popup window (not a Silverlight popup). Silverlight can create the popup window via calls to Javascript.
The 2 Silverlight instances usually need to communicate data, but there are several solutions to that problem too. Javascript can also talk to Silverlight objects (love that feature) and pass information back or you could simply pass data one-way via URL parameters.