The solution was found here:
I am repeating on my blog simply for redundancy purposes because the above article was the only source I found to the problem.
If you get the following Build error:
Error loading PostSharp symbol file: Access to the path 'C:\..\SomeFile.pssym' is denied.
Likely cause: you deleted solution binaries in order to clear out lingering out-of-date assemblies. Unfortunately, the PostSharp process becomes confused and prevents you from building the model assembly.
Workaround: Close and re-open the solution.