The understood policy is that you have to actually be sick. But everyone plays the game. There've been times when HR just let me use a few sick days over xmas holiday instead of taking unpaid days though. They are generally "cool" about that sort of stuff.

As a company they can't say "hell yea! take off a free day when you don't feel like coming in and just use a sick day!", but they don't require doctors forms or anything. It's an honor system type thing.