I have heard that HDMI performs a "handshake" between all components connected, and defaults to the settings supported by all the components: i.e., the DVD player supports 5.1, the receiver supports 5.1, the plasma supports stereo souns, so the HDMI won't pass the 5.1 signal to the receiver since the TV can't use it. Now, I only heard this on Thursday from a friend who read it in the online "Perfect Vision", so take it with a grain of salt-I've only ever used separate audio and digital lines.
Bryan...any custom installers on the forum want to chime in?