I have been reading a bit about ion sensing ignition systems, and this stuff is fascinating. I heard about it first from reading on the Megasquirt forums. Apparently there are a few people experimenting with this and working on a solution that would work with megasquirt EMS. Saab is supposedly already using the technology in some of their cars now.
Here are some links for anyone interested.
Basically the spark plug is used as a sort of sensor that can tell at what degree the peak cylinder pressure is produced, and the spark timing can be adjusted on the fly to optimize power and efficiency from this information. It can also be used to detect misfires.