Here is an interesting article about fair use and documentaries.
"Documentary films are an important source of education, commentary and criticism," Media/Professional president Leib Dodell said. "Rigidly requiring licensees or releases in all cases does not give filmmakers the flexibility to take advantage of fair use in appropriate situations. This initiative makes fair use work in the real world of independent filmmakers."
Documakers getting aid on Fair Use, The Hollywood Reporter, February 23, 2007
For additional information on this topic, see this post from the Info/Law blog.