Runtime Flow v1.1.3 adds ability to monitor Silverlight 4 applications. A Silverlight 4 application runs in a browser and to start monitoring it you should specify your preferred browser and a startup page in the Customized Run dialog:
Internet Explorer Customized Run settings
An Silverlight application in Internet Explorer
Firefox Customized Run settings
An Silverlight application in Firefox
Runtime Flow of the Silverlight application
There is one complication when using Internet Explorer in Windows Vista. When started, Internet Explorer creates separate low integrity process to display contents and exits the original process, stopping Runtime Flow monitoring. In this case you should uncheck the Stop monitoring when the command completes option and use the Stop button on the Runtime Flow toolbar (it is not necessary in Windows 7):
The new Stop monitoring toolbar button
Clicking on a function in Runtime Flow currently doesn’t go to the source for Silverlight projects as the Silverlight runtime provides slightly limited information.
Runtime Flow blog (Understanding a control flow in a running .NET application) 