That said, you don’t even need an explicit signal because it is easily implied. There are HDMI switches that will detect activity and switch to that port. The better ones even have a priority setup based on input port, so you can handle the case of multiple devices on at the same time. I don’t know if any stereo receivers or TVs have the same feature, but it would not be a difficult one to implement. This is not even a new concept as many switches of older analog devices with component/composite connections will do this as well.

In the end this type of feature is mainly just a function of software on the receiving device. Auto power off does exist on some TVs and most projectors when all input signals are inactive for a period of time. You are right that the complex remotes to automate your theater might not like it, but with the right consumer demand there is nothing preventing the makers of TVs and receivers from making things easier.

Do you know about the SCART cables from the days of standard definition TV? Here in Europe, most VCRs, DVD players and whatever are connected with those (http://en.wikipedia.org/wiki/Scart). Wikipedia says they aren’t really used in America. The connectors are bulky and impractical, you can’t figure out which way to put them without looking, they keep falling out halfway and you end up with a yellow picture or no sound. But when they do work, they’re actually pretty nice. They have video and audio in and out and can carry composite video, RGB, S-Video and some other stuff.
And they have a wire telling the TV when a device is powered on. So the TV can automatically switch to VCR, DVD or satellite box input when you turn the respective devices on. The wire should also tell the TV whether the image is 4:3 or widescreen, but I’ve never seen that feature used. And since most game consoles have special cables (many of them using RCA jacks and the RCA-to-SCART-adaptors you see in the wikipedia article), automatic switching doesn’t work with those.
So the only thing SCART can do is switch from the built-in receiver of the TV (used mostly for cable TV over here) to a VCR or DVD player and back. No automatic power on and off or anything, but at least somehow like what you described in your article.