Upon upgrading SSO in vCenter, I ran into a failure during the process. The process would fail halfway through and hang at configuring SSO Components (shown below). Upon the failure and investigating the msi log, it was showing a DNS type error.
This only affects you if you are upgrading from 5.1. Anyone upgrading from 5.5, 4.x or a fresh install of 5.1 update 1a or later will not experience this issue. And obviously, does not affect you if you are doing a fresh install of 5.5.
The fix is actually rather simple but confusing by the wording. The change is made in the registry at the following key:
HKEY_LOCAL_MACHINE\Software\VMware, Inc.\VMware Infrastructure\SSOServer\FQDNIp
If the value inside this registry key is an IP, you must change it to the actual FQDN of your SSO server. For me, mine was on my vCenter server, so it was set to its FQDN.
The actual key name is FQDNIp, but it needs to be the name. Hence the confusion!
More information can be found in the following VMware KB.