Updated: January 24, 2014.
I have read a few posts within the community lately [1][2] about an issue being encountered after a fresh install of vCenter 5.5. Basically, what happens is the only user that can log-in to the vCenter server is the SSO administrator (Administrator@vsphere.local). This may seem like unexpected behaviour, especially considering that even the account (perhaps an AD service account) that was used to perform the install is also unable to log-in and see the vCenter Server.
In contrast to the long title for this post, the solution that I would like to share is short and straight forward. I hope that this solution will save you from some post-install headaches.