Saw a cool article cross my RSS reader today on How to enable passthrough authentication in VMware VirtualCenter 2.5.
I just tested it; it works.
Source: http://www.virtualization.info/2008/03/tech-how-to-enable-passhrough.html Thank you Stuart Radnidge and Alessandro Perilli!
To use it, simply add -passthroughAuth -s vchostname to the end of the shortcut used to launch the VI 2.5 client.
By default it uses the Negotiate SSPI provider, however since they have fully implemented the interface you can change that behaviour to use Kerberos by adding the following within the node in the vpxd.cfg file on the VC server: