Well, thats exactly the point and it was mentioned before. The whole thing is meant to be cost effective, so who would invest in a backpane that serves no purpose? I mean people who would get such a card really dont care if they have to wire them up seperatly onto a usb hub for 5 bucks or on a backpane that will be 50 bucks or more, I'm pretty sure they'll go for the cheaper alternative, it the backpace doesn't add any kind of feauture... The only reason to design a backpane, would be if it had some additional functionality, nobody is going to buy it, if its just a nice way to stack up the doughterboards.
And as far as I see it, that fuctionality would be Ethernet!

I cant imagine implementing an ethernet controller is too hard for people that can programm FPGAs, for example for Arduino (which was mentioned before and although feautures SPI, I2C etc) there are readily available schematics and libraries just for that purpose: http://www.practicalarduino.com/freetronics/EtherTen-schematic-worksheet.pdf

btw, I have been following this thread from the beginning, but since I am only in my third semester of electical engineering, I really dont have to much to say about most of the specs you're discussing here, please dont get me wrong here, I really apreciate your effort but i really think you should consider to develope a backpane that gives people a reason to buy it in the first place.