An adult Eared Grebe in it's non-breeding plumage.
When I was at Green Cay there were several of these birds hunting near the Nature Center. When I returned a few days later with a bigger lens they were gone. Probably just passing through.
From the web, this is the most abundant grebe in the world. But it is found mostly in the Western United States.
12/01/06 Update - Green Cay lists this bird on their picture board as a Horned Grebe. However, after more research on the web, I remain convinced that it is an Eared Grebe.