That was me who discussed a civilization that had nothing to consume but other humans. Thanks for bringing it up, but I had already dealt with that in the stories as soon as someone pointed out that problem when I was much younger (turned out to be easier to fix that I thought), but telling what the solution is would give away too much, and since I might actually be able to get these published now that cannibalism is not nearly so taboo as it was back in the 80s when I first tried to submit them (Zombie movies were not nearly so prevalent then as now). Once the solution is revealed... It makes for another grim and, some have said, twisted surprise.
I too have wondered about the whole matrix thing. There are some very good arguments against it, which I tend to give more weight to than the arguments in favor. Yet, the arguments in favor did not take into account the waste generated by the humans being used to support the generation of power, nor did they take into account any possible superconducting tech the AIs may have had. I cannot recall if any of them took into account that the AIs were also not using every human farmed as a battery, but were using many of the farmed humans as food for the living humans. There is also some evidence from the games that the AIs were also using algae as a supplement for the human batteries.
Also, on the point about people donating their bodies to cannibals when they die... I have often thought that it would be a horrible joke for some cranky rich old guy to play on his heirs to make them eat him if they wished to inherit any of his fortune.
Sod that, start a religion in which people have to symbolically eat your body and drink your blood once a week. Better yet, tell them that when they do it, it magically becomes the real thing!
As promised, here is the "Q" part of the Less Wrong Video Q&A with Eliezer Yudkowsky.
The Rules
1) One question per comment (to allow voting to carry more information about people's preferences).
2) Try to be as clear and concise as possible. If your question can't be condensed to a few paragraphs, you should probably ask in a separate post. Make sure you have an actual question somewhere in there (you can bold it to make it easier to scan).
3) Eliezer hasn't been subpoenaed. He will simply ignore the questions he doesn't want to answer, even if they somehow received 3^^^3 votes.
4) If you reference certain things that are online in your question, provide a link.
5) This thread will be open to questions and votes for at least 7 days. After that, it is up to Eliezer to decide when the best time to film his answers will be. [Update: Today, November 18, marks the 7th day since this thread was posted. If you haven't already done so, now would be a good time to review the questions and vote for your favorites.]
Suggestions
Don't limit yourself to things that have been mentioned on OB/LW. I expect that this will be the majority of questions, but you shouldn't feel limited to these topics. I've always found that a wide variety of topics makes a Q&A more interesting. If you're uncertain, ask anyway and let the voting sort out the wheat from the chaff.
It's okay to attempt humor (but good luck, it's a tough crowd).
If a discussion breaks out about a question (f.ex. to ask for clarifications) and the original poster decides to modify the question, the top level comment should be updated with the modified question (make it easy to find your question, don't have the latest version buried in a long thread).
Update: Eliezer's video answers to 30 questions from this thread can be found here.