I applaud the implemanation of multi-channel surround options within MIR. I would like to see, however, a single industry standard for the speaker type and positioning for mixing and playback of multi-channel content. For a seamless 360 degree, holosonic, image specific soundfield, 5 to 7 identical (with and optional height channel), front radiating (non dipolar) full range speakers (at ear level) is the optimal approach. But if speaker positioning is one way when the music is mixed, and another upon playback, the multi channel mix be will a distortion from how it was initially concieved.
Your suggestion is an excellent one for 7.1 systems. 5.1 already has ITU-R Recommendation BS.775-2 (07/06), which specifies speakers at 0°, ±30°, and ±110°, although it is unclear how many people actually have their speakers laid out this way. (Four of the speakers in a square seems to be more popular.) Ambisonics, however, is fundamentally different from 5.1 and 7.1, and does not need a standard speaker layout. This is one of its great advantages.
What is encoded in Ambisonics is not speaker feeds, but direction. When mixing in Ambisonics, the positions of the speakers are unknown and are of no interest. Further, when Ambisonics is decoded to speaker feeds all of the speakers cooperate to localise a sound in its correct position so, for example, when the speakers on the left push those on the right pull. The speakers all contribute to the creation of a single coherent soundfield.
For more information on Ambisonics, please see [url=http://www.wikipedia.org/wiki/Ambisonics]the Wikipedia page[/url].