I would just like to add one more thing that might be of help to some folks out there. Since I've already gone insane trying to figure this out, hopefully might comments might help someone else from following in my footsteps. 🙂
I just got off the phone with Marantz. Marantz is the maker of my surround-sound receiver. The person I spoke with was extremely knowledgable, and we ran a few tests to see if the information that AppleCare gave me was correct or not. I hate to say this, but AppleCare was not correct regarding the HDMI/TOSlink cable information. Here is what I discovered:
When I plug my Apple TV into the back of my Marantz surround-sound receiver (using the HDMI cable), it DOES allow 5.1 surround-sound to happen! This is the opposite of what AppleCare told me. They told me that I had to go out and purchase a TOSlink (Optical Audio) cable, and connect that to the back of my Marantz receiver in order to receive 5.1 surround signals---and this is wrong. (In one of my earlier posts, I said that AppleCare told me that you CAN use the HDMI cable for surround-sound if you'd like to, however, you can NOT run it directly to the receiver. They said you have to run it first to your TV, and THEN to your receiver in order to make 5.1 work).
So it turns out that the common sense answer is correct after all. If you run the HDMI cable from your Apple TV to your surround-sound receiver, that is just fine and you will get 5.1 sound. I just did it, and it works perfectly. I rented a 5.1 surround-sound episode of "The Simpsons" (for 99 cents, as another person suggested earlier), and it worked great----WITHOUT the TOSlink (Optical Audio) cable. Since the Marantz receiver self-detects the audio signal, then according to Marantz, that means that the previews I was looking at (previews of TV shows and movies in the iTunes Store) are only in stereo, even though the actual program itself is in 5.1. So don't expect to have preview clips in 5.1 surround-sound----you should only expect them in stereo.
I hope this makes sense. Please feel free to ask me any questions, and I'll do my best to answer. I've spent days on this thing, and I just wish that AppleCare would have provided me with the correct answer to begin with. That would have saved me a lot of trouble.