Although much of the administration of servers running VMware ESX Server 3.0 will occur in the Windows-based Virtual Infrastructure client connected to a VirtualCenter server, there are times when it is quicker or easier to perform an administrative task directly on the ESX Server itself—either via the command-line interface (CLI) or via the VI client authenticating directly against the ESX Server. The problem with this is that, by default, administrators will have to use different credentials when connecting the VI client to ESX Server directly. In addition, these credentials must be managed separately from Active Directory, and separately on each individual ESX Server. As the number of ESX Servers in a farm grows, this can quickly become an administrative nightmare.
Read the complete article here: http://blog.scottlowe.org/
http://www.vmadmin.co.uk/index.php/resources/35-esxserver/55-esxadauth