LOL! fireice never watch the devil's advocate?
anyway, the devil is probably a metaphor referring to Miranda Priestly (Meryl Streep), one of the city's biggest magazine editors in the movie, who happens to be very mean and demanding.
to better summarize the story, it's about a beautiful young woman (the lovely Anne Hathaway) who, fresh from college, gets a job working as a personal assistant to Miranda and how she struggles to keep up with her boss's capricious and demeaning requests.
and no, there's no supernatural stuff in this movie.