Just happened to see this article:
http://www.smallnetbuilder.com/nas/nas-howto/31828-fun-with-rsync-and-more-on-the-wd-my-book-live
It claims the NAS box ships with Debian on it and is completely rootable.
I don't have any first-hand experience with it.
--
Eric Cooper e c c @ c m u . e d u