Actually there are new standards for both 6.1 and 7.1, but only for home theater and only a few DVDs take advantage of it yet. The 6.1 standard adds a rear center channel to complete the "surround" effect. The THX specification actually calls for *two* rear center speakers, so decoders that follow the THX standard are designated "7.1". See "www.ovation-av.com/ov2000/theater_ed_14.htm" for an explanation of this.
I'm not sure why THX asks for two rear centers yet only one front center. I think the reasoning is that when a sound is *directly* in front of or behind you, your ears are unable to perceive which direction the sound is coming from (because our ears point sideways, they get confused). To alleviate this, I think Lucasfilm may have specified two rear centers to provide a slight spacing effect so the sounds come into our ears at an angle from behind and thus immediately let us know that the sounds are behind us.
Basically, in computer gaming 6.1 and 7.1 will remain a non-issue for at least a couple more years. By then we may start seeing sound cards that support these additional formats.