I didn't know what section this belonged under, so put it here. If this is better elsewhere please move it.
I recently found out about a magazine called Makezine, (link)
http://www.makezine.com/ , or "Make:" in paper form. I found an interesting article in volume #15 about how to use inexpensive FM Walkie-Talkies as a camera trigger. I couldn't find the whole article in one file online, but here's links to the necessary pages from some kind of magazine preview thing. If you just close the "purchase options" popup that happens you still get to see the full article with schematics and stuff.
(page 1)
http://www.make-digital.com/make/vol15/?pg=153(pages 2 & 3)
http://www.make-digital.com/make/vol15/?pg=154(pages 4 & 5)
http://www.make-digital.com/make/vol15/?pg=156This would be great for kite and balloon aerial photography too, KAP, instead of just running an intervalometer you could choose when to trip the shutter.
I happened to buy a couple of these FM Walkie-Talkies with only two channels, A and B, for about $5 from a surplus place a few years ago. You can get them really cheap if you hunt around. On mine they have the call-button and all. The receiving unit emits a warbling note when it's pressed on the transmitter. Is there an easy way to modify this idea for CHDK cameras? Something much easier than what's in that article? Since they have the squelch adjustment on them, could that be turned up to keep the speaker output totally dead, and then the voltage from the call-signal to the speaker could trip a relay in place of the speaker, or something like that?
On further thought, these Walkie-Talkies also have a headphone jack. Could this be accomplished without even opening up their cases? Just plug in some kind of simple circuit that detects the sound output on the headphone jack and closes a switch circuit. That'd be sweet.