I upgraded an older project from vs2010 to vs2012 this morning and debugging broke. When attaching to the web server process this error appeared:
"no symbols have been loaded for this document".
The solution was easy, I switched the default code type from "Managed (v4.5,v4.0)" to "Managed (v3.5, v3.0, v2.0)".
(I'm not sure why vs2012 can't look and see that my project is running as .Net v3.5 and just automagically set it.)