When debugging an addin with VS2010 the explorer is started in normal mode. As soon as the addin is loaded, it is switched to managed mode and VS2010 stops at the breakpoints.
With VS2012 it is different - when the addin is loaded, explorer.exe is not switched to managed mode and breakpoints are not hit.
But when I turn off VS2012 debugging, go to the folder and ensure the dll is loaded (so the explorer is switched to managed mode) and attach VS2012 to it - then the breakpoints are hit.
I use VS2010 Express and VS2012 Express on Win7 Pro x64.
I posted it on stackoverflow: http://stackoverflow.com/questions/13015580/visual-studio-2012-express-debugging-explorer-exe-no-breakpoint-hit
More detailed information: https://connect.microsoft.com/VisualStudio/feedback/details/769118/