It is actually 3 signals: clock, data, and an enable signal. But with with only one signalling wire, you can easily deduce that it is a serial format. If someone had a logic analyzer we could get on it to find out if is a standard serial format.

Here is the pin outs (with the data signals listed):
http://forums.audiworld.com/tt/msgs/63403.phtml

Also note at the bottom of that page there are some more links to schematics. They dont mean much to me, but they might help someone decode what it would take to interface an empeg.