“Ever since vSphere 5.1 launched we’ve been getting a fair number of support calls about Single Sign On, or SSO for short. It is a requirement now, and many of you are getting caught on some aspect of the upgrade/implementation.” – VMware blogs site

Enjoy… 

1. http://blogs.vmware.com/kb/2012/10/vsphere-sso-resources.html
2. Don't install 5.1, install vCenter Server 5.1.0a (or above)