There's an interesting article in the Globe and Mail this week re: how difficult it is for documentary film makers to clear copyrights. As the article (quoting Robert Stone of an American University study) aptly puts it:
"'Why do you think the History Channel is what it is? Why do you think it's all World War II documentaries? It's becuase it's public-domain footage. So the history we're seeing is being skewed towards what's fallen into the public domain.'"