and apparently there is a way to do it if the sound card can record in "bit accurate mode" (according to this thread):
I've used a USB/SPDIF box to do this. It had only SPDIF in and out, and nothing else - boxes with more inputs may or may not be "bit accurate" (the box suffered from occassional jumps, failing to record for a fraction of a second, but it only took two or three goes to record an entire side accurately).
David