I have some strange thing happening in my application. I was able to debug anything but silverlight code !!
I did so much of RnD on net and tried various tricks, but only this thing worked : -

When you add a Silverlight project to a asp.net solution, you create 2 projects…
A. the startup project (interface starter)
B. the interface itself

1. right click on the startup project (SilverlightWeb — the one with default.aspx in it )…
2. click on ‘Property Pages’
3. open ‘Start Options’ Tab
4. enable Silverlight Debugger
5. OK

This was the solution for me