I recently did some work in Eagle (Cad software for PCB's) to create a library for the API500 (http://www.apiaudio.com/5006b.html) standard.
This standard looks to be an awsome standard for DIY'ers to standardise around, although I do have some concern for it's ability to deal with digital audio.
For instance, the only supplies in the standard are ±16VDC supplies. This is obviously aimed at the analog signal chain supplies. Not knowing enough about noise in digital and analog systems makes me concerned that a digital product could really spoil the supply, not only in the card that contains the analog and digital, but also the rest of the box (the other 5 units in the lunchbox).
Such "analysis" of the card and it's format really made be re-evaluate the motherboard plan that I was doing, but also look for an alternative sub-rack system. A post on group-diy pointed to something called a Frac Rak - made by Paia Electronics. http://www.paia.com/fracrak.htm
Someone mentioned in the thread that the Paia board wasn't strong enough in comparison to the API lunchbox. However, a little more research into the Fracrak shows that it is incredibly popular in the DIY Synth market. Those folks are pretty much doing what we're doing here, except that their boxes are more fixed in place (at the geeks home) :-)
I kept on chewing onthis thought, wondering on how I could strengthen the box, how I could make it so that card edge connectors could remain in place, even when the unit is under stress.
The conclusion I came to was that if you were really serious about making the unit mobile, you'd flight case it on its own (in an SKB like case). AS for card edge connectors... forget it. I'm still tempted to use ribbon connectors with 0.1" spacings. That way, if these is some bending, the card mounted vertically (e.g. the mic amp) isn't going to put pressure on horizontal board (the motherboard).
In the case of the system that will mainly be individual cards that can have audio chained from one to the other, then there isn't so much of a need for a control processor or CPLD. The main use for a motherboard system of such a kind is in the distribution of shared resources - power, clocks, control (SPI and I2C) etc.
My design experience in these fields is limited, however, I have seen many implementations in Pro Systems, and asked many a question. Any comments you have are appreciated.