This can come from a number of potential errors with XAML. The first thing I now check, is to see if I am attempting to add multiple FrameworkElements with the same x:Name. For me, I was dynamically adding usercontrols to a canvas, but I made the mistake of giving the root element of the usercontrol an x:Name.

 Other cases where I created this error:

• Specified a Grid.Row or Grid.Column that was greater than the actual number of columns or rows

Read the complete post at http://silverlightaddict.com/blogs/learningsilverlight/archive/2008/06/10/silverlight-2-beta-2-value-does-not-fall-within-the-expected-range.aspx