Today I downloaded RhinoMocks and referenced in my test project (Microsoft Visual Studio Testing Environment) and executed my unit tests.
My tests failed with the same error (for all tests). Visual Studio was kind enough to find that the RhinoMocks.dll which I downloaded is not trusted and cannot be deployed in a test environment!
(click to enlarge)
It appears to be that any dll you download from the Internet or get it via email, some extra information called AES gets attached. This cautions the user that it was downloaded from Internet and needs attention. More info on this here
To resolve this issue, just open the properties of the file and Unblock it :)
This corresponds if you have Windows XP SP2, Windows Server SP1 or higher