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.'"
You can read the whole article on-line at the newspaper's website, but only if you are willing to give them your credit card. (Alternatively, you can click here for a pdf. Shhhh.)