I had this idea about a month ago, but I was too busy prepping for MIX to put it together.  Take SilverlightPad, add a javascript textarea, and a run button.  When the button is clicked, dump the XAML and JS into a new iframe and create a new instance of the Silverlight plug-in to run the code. 

To invoke this, click on the JS tab and then the "parse" button will become a "run" button.  This interface is a bit clunky, but it's servicable for now.  Each scene has a simple script that handles the first element's click event.  The button scene has a more complicated script which triggers animations on mouse events.  As for code editing, there's one limitation in that you need to have a JS class defined named "test".  In a future update I'll add an options pane where the name of the class this can be specified (or omitted). 

This update also uses the mouse cursor feature, so interactive UI parts now change the cursor.  Click the image below to run the sample.  The source for this sample can be found here.