We recently upgrade to vCenter Server 5.1 and had an issue with our XenDesktop Desktop Controllers. Version 5.1 significantly changes the authentication method with vCenter by breaking out authentication to a separate service called the vCenter Single Sign On service.

When viewing the information in the XD Desktop Studio the host connection to our vCenter server had a red X indicating there was an issue connecting to the host. The XD Host Service was started so that wasn't the problem.