Hi, 
I have been using dbrous's  printing sample  and customized it in my applications, it is always great. So here, I want to say I really appreciate his work.
Yesterday  by an acciden, I found  the control had a problem with the GraphicsLayer which has clusterer renderer, for all other graphicslayer are OK, as soon as I removed clusterer renderer, it was OK.
The problem caused the application collapsed, when you open the print control,  the graphics only show either the app ( main browser ) or printing control ( Cloned map) , after toggles a few times, then the app was collapsed, the error showed as the the attachment.
The issue might still related to  ArcGIS server Silverlight API ( about clusterer renderer). If somebody had the same problem and fixed, please let  me know, because I still want to use the clusterer for the GraphicsLayer and the printing control.
Thanks.