I'm not sure it's even a fork. It appears to plug a different backend/frontend into the nix structure, so it appears to be a "patch", for want of a better word. (at least that's what I gleaned from http://www.fdn.fr/~lcourtes/software/guile/guix-ghm-2012.201...)
(More info here: https://lists.gnu.org/archive/html/guile-user/2012-07/msg000..., taken from icebraining below)