>When in spot metering, the exposure will be set so that the >one spot is medium gray. Matrix and CW metering just use a >larger area for averaging to medium gray. >
That's my understanding of spot metering as well.
Matrix metering: I recall reading somewhere this was actually quite nuanced in operation. I believe that the author mentioned that there was something done with face-prioritization and/or skin tones, so that matrix metering would sacrifice other elements in the scene to get a proper exposure of the skin / face.
Thom Hogan's book has a nice writeup on matrix metering, and how depending on lens type, matrix metering behaves differently - for example, there's lenses and situations where the upper portion of the frame is ignored, as the camera has decided it's just sky, and not important. That's a huge, gross, oversimplification of what Hogan wrote, just to be clear.