I just hit this problem yesterday too. I can use F10 (step over) and F11 (step through) in other projects, such as a C# console app that converts an XLS to a featureclass but not in my Silverlight project.
I know this is kind of beyond the scope of Esri, but any help would be appreciated.
Running Win7 64bit, VS 2010 (installing SP1 now), Silverlight API 2.3, debugging in IE9
Thanks, TG