An update for the vSphere vCenter Server 5.1 has been released to address a couple of issues that didn't make it into the GA version. If you have experienced issues with timeout errors occurring when logging in to the vSphere Client or logging in to a vSphere Web Client as a user of an Active Directory domain that has a non-standard UPN or have changed the Single Sign-On master password you probably should look at updating to the new release.
You can get the updated files from vmware.com and I would highly recommend reading the release notes for a full list of known issues that have been resolved in this release as well as a readme that has been included to add additional assisting information.
If you haven't been sold on the vCenter Appliance yet you should checkout the update process when using the vCenter Appliance; just navigate to the upgrade tab and apply the update from there (internet or cdrom), the process for updating the vCenter appliance is very slick and another reason why you should be looking into it. For those using the installable version of vCenter Server (Windows) we provide an ISO image.