User must have deployment administrator rights to register plugins,
http://blogs.msdn.com/b/crminthefield/archive/2011/08/17/error-message-assembly-must-be-registered-in-isolation-when-registering-plugins-in-microsoft-dynamics-crm-2011.aspx
http://blogs.msdn.com/b/crminthefield/archive/2011/08/17/error-message-assembly-must-be-registered-in-isolation-when-registering-plugins-in-microsoft-dynamics-crm-2011.aspx