i think they've been planning to for a while:
http://code.fabfile.org/issues/show/19i haven't really followed their progress too closely, but they seem to be close to releasing something for 1.0.
I think they do client-side threading while i prefer to to do fork() on the server to avoid multiple ssh connections and retain maximum control.
i'll reevaluate when they ship. till then parex works for me.