Ya know I had the exact same question when I was ordering my new HT receiver (it has HDMI switching). I feel bad, I call crutchfield with no intent of buying from them, but at least they seem to know the answers.
I worded it like this: Since my new receiver will have HDMI switching, does the switching just pass-thru the signal or does it decode the audio. Do I still need my optical cable for the DD?
The response I got after the guy put me on hold "To ask a tech to make sure" was that yes, the signal will pass-thru, but the receiver will also decode the audio, so I would have to mute or turn down the volume on the TV if I want to do the full HT setup. That's what I wanted to hear, but I will believe it when I see it.
Hopefully someone here will report back on some experiences, otherwise my receiver is due to be delivered tomorrow so if no one has responded yet, I'll post my findings.
Edit:
Well, it appears from what I have been reading today is that for the single HDMI cable to work your receiver needs to have HDMI SWITCHING, not just HDMI pass-thru. The receiver I ordered has HDMI switching so I should be good to go, but I'll still report back.
Man, it's been a while since I upgraded my old receiver. I'm never gonna let myself fall this far behind (informational wise) again.