"Furthermore, it's impossible to come up with a CF/SD card that would have direct sound feeding via the card bus to the inner hardware – the current card buses just don't support this kind of functionality. "
I have to disagree with this statement.
http://www.core-sound.com/pdaudio-cf.html
Core sound makes a SPDIF CF card that can record up to 24 bit 96khz. It's used by a lot of pros in the industry.