It is so simple! Silverlight contains a class, System.Windows.Browser.HtmlPage, that allows access to the Browser's Document Object Model. It has a method called PopupWindow, that allows a Silverlight developer to open a Popup Window from inside Silverlight.
A sample code can help understand this better:
HtmlPopupWindowOptions options =
options.Directories = false;
options.Toolbar = false;
options.Status = false;
options.Menubar = false;
options.Location = false;
options.Width = 400;
options.Height = 400;
options.Top = (Convert.ToInt32(HtmlPage.Window.Eval("screen.height")) - options.Height)/2;
options.Left = (Convert.ToInt32(HtmlPage.Window.Eval("screen.width")) - options.Width) / 2;
HtmlPage.PopupWindow(new Uri("http://www.silverlight.net"), "_blank", options);
Notice the bolded words above. The HtmlPopupWindowOptions class allows you to set options for the Window that is to be opened. For example, show/hide the Toolbar, Menubar, etc ...
The HtmlPage.PopupWindow is the method that you can use to show the Popped up Window. It takes as input, a URI representing the URL to open, the target of the Popup Window and finally an instance of the HtmlPopupWindowOptions.
That's it. Now you have a Centered Popup Window triggered from inside Silverlight application.
Hope this helps,