A while ago there was some talking on the FaD forums about a "last WU/last update" being send out to all clients that'll uninstall or at least shut down FaD but I haven't heard anything about it since. The prob was that an uninstall process in Windows requires user action, clicking yes. It's meant as a security thing.
Other than that, I guess the client will just sit there forever, as jsut another background process, if nobody uninstalls it.