Typical SharePoint 2010 install. I had created all my domain users, set up my servers, and started installing the prerequisites. When the first box in the farm restarted and the preparation utility came up, I was asked for credentials and provided them, only to be rejected. At first I thought it was a standard "access denied" error, but a look at the fine print revealed the error titling this post.
I tried Run As Administrator; still no success.
Eventually I realized that I was signed in as the farm account, though I had not yet made it a local administrator. I right-clicked Computer Management, ran THAT with the proper credentials, made the farm account local admin, and all was right with the world.
Sometimes it's the little things.