While perusing the web, I stumbled across a nice "experimental" feature of the VMware VI 2.5 client that allows you to use your logged on Windows credentials to automatically log onto VirtualCenter.  OK, so this may not be big news for everyone and it's not like this is a huge secret, but I find it quite handy.  To try it out, create a shortcut to the VMware Infrastructure Client (or modify the default one) to include the following bold text in the target field:

"C:\Program Files\...\VpxClient.exe" -s VCServerName -passthroughAuth

...where VCServerName is the name or IP address of the VirtualCenter server.

When you launch the VI client, the logon window looks like the following, and you are automatically logged into VirtualCenter: 

Although VMware labels this as "experimental", I doubt there is any serious support issues in your future for using this.  I would guess your worst-case scenario may be a failure to connect, but even that's highly unlikely.  This feature will most likely end up officially supported in VMware VC 2.5 Update 1 when released in Q2-08.