I guess what it comes down to is common sense. For non profit fan art type things, (Or even reletively low profit, as I am certain hollywood types would consider Poser characters, or artwork auctioned at cons etc...) Most copyright holders turn a blind eye as long as you don't step on their toes by infringing on their serious profits. The Troopers mod for Unnreal tournament hasn't been "foxed" yet becaus Lucasarts knows it isn't likely to affect the sales of their games. In fact being too hard on fans can create hard feeling and theoretically HURT their profits.