I have just performed the above upgrade, to vCenter 5.5 (Windows) and am struggling to get AD authentication working.
After the upgrade the identity source relating to the Windows domain was present in the web client, and there were users listed. Unfortunately the list contained exactly 200 users from our AD (out of 2000) and did not contain any of the admins that need, and used to have, access to vCenter.
I added a DSN to the identity source and made it the default, but nothing has changed: integrated AD login fails.
I downloaded vCenter 5.5 several weeks ago so may not be on latest version, but I'm reluctant to upgrade UM until I cna get it to connect to VC using Windows creds.