got a friend who does it all the time. He's actually huge into sports and knows his stuff. But he doesn't necessarily have favorite teams. Came up last year in the NFL playoffs as he was cheering for any canadian team to win... I was cheering for the Blackhawks. Had to smack talk each other during the canucks/hawks series and after the hawks won they were his team in the west since there weren't any canadian teams left.
as for NFL... his "favorite" team is the falcons... although he's not diehard or anything. Once we beat them he's cheering for us now. Doesn't bother me that much cuz I'm the only pack fan of my friends (colts, bucs, falcons, seahawks, vikings (bro&dad), 49ers, and dolphins among them) and I got the last laugh this year.