Over at the Science Fiction/Fantasy writing section of the Absolute Write message board, one of the posters proposed a classification system for urban fantasy. As someone with a BS in Biology, I couldn't resist modifying his a bit.
Kingdom - SF (Speculative Fiction)
Phylum - Fantasy
Class - Urban
Order - Contemporary, Historical, Futuristic
Family - Romance, Mystery, etc.
Genus - demons, fey, shapeshifters, magic users (witches, psychics), angels, vampires, etc.
Species - author's specific interpretation of the supernatural creatures and cultures