VMware has released vCenter 5.1 U1A which resolves a critical issue with vCenter Sign-on in multi domain environments. I noticed this issue first hand in a lab environment with several domains when trying to authenticate users.
The original issue is outlined here but in short if you have a user in a multi domain environment in many groups you may receive failed logins in your environment. Whether you have experienced issues or not this brings to light the critical dependencies of SSO in your environment and the need to understand its functionality and properly prepare for backing it up and restoring it.
You can check the release notes for 5.1 U1a here. Note that 5.1 U1 has been removed from download and this update has replaced it.