Well, just the other week, we had the 5-0 Patriots at the 5-0 Cowboys...
I think this morning on ESPNews they had a little blurb about this (since 1924). I think the Vikes and someone else (LA Rams?) were in this situation around 6-0 back in the 70's. I think they said it's happened just 4 times (including this week's game).