Why is the Guy Fawkes face used for the Occupy protests?

The mask was worn by the protagonist in the graphic novel, and later film, V For Vendetta to obscure the character's physical scars, emotions and, of course, to protect his identity.

V battles against a totalitarianism government, and early in the book (and the finale in the film) destroys the Houses of Parliament by blowing it up, something Fawkes had planned and failed to do in 1605 and hence the connection.

The Occupy Movement are loosely associated with Anonymous and needed an all-purpose image to hide their identity and act as symbol that they stand for individualism.