As of 10/31/2013 VMware release vCenter 5.5.0a. There are no other corresponding release that came out with this version. It looks like this is a patch to vCenter dealing with log on issues.
The full release notes can be found here.
Issues resolved with this release are as follows
- Attempts to upgrade vCenter Single Sign-On (SSO) 5.1 Update 1 to version 5.5 might fail with error code 1603
- Attempts to log in to the vCenter Server might be unsuccessful after you upgrade from vCenter Server 5.1 to 5.5
- Unable to change the vCenter SSO administrator password on Windows in the vSphere Web Client after you upgrade to vCenter Server 5.5 or VCSA 5.5
- VPXD service might fail due to MS SQL database deadlock for the issues with VPXD queries that run on VPX_EVENT and VPX_EVENT_ARG tables
- Attempts to search the inventory in vCenter Server using vSphere Web Client with proper permissions might fail to return any results
- vCenter Server 5.5 might fail to start after a vCenter Single Sign-On Server reboot
- Unable to log in to vCenter Server Appliance 5.5 using domain credentials in vSphere Web Client with proper permission when the authenticated user is associated with a group name containing parentheses
- Active Directory group users unable to log in to the vCenter Inventory Service 5.5 with vCenter Single Sign-On
- Attempts to log in to vCenter Single Sign-On and vCenter Server might fail when there are multiple users with the same common name in the OpenLDAP directory service
- Attempts to log in to vCenter Single Sign-On and vCenter Server might fail for OpenLDAP 2.4 directory service users who have attributes with multiple values attached to their account
- Attempts to Log in to vCenter Server might fail for an OpenLDAP user whose account is not configured with a universally unique identifier (UUID)
- Unable to add an Open LDAP provider as an identity source if the Base DN does not contain an “dc=” attribute
- Active Directory authentication fails when vCenter Single Sign-On 5.5 runs on Windows Server 2012 and the AD Domain Controller is also on Windows Server 2012