Whilst installing a TFS & CCNet demo system I got an exception
System.Security.SecurityException: That assembly does not allow partially trusted callers
when I tried to load the CCNet WebDashboard.
The problem was that default CCNet installer had created the WebDashboard on the default web site as the virtual directory
http://localhost/ccnet
As this was also a TFS server the default web site was a WSS3 server. Bascially the SharePoint did not like (trust) the CCNet virtual directory's DLLs.
The fix I used was to create a new website and point it as the WebDashBoard directory so it could be accessed without effecting the SharePoint e.g.
http://localhost:81
I could also have edited the SharePoint's web.config to trust the CCNet DLLs