http://www.wi-gear.com/products/imuffs/mb210specs.php

BT headphones with controls on them. Not exactly what you want, but you could probably hack a switch/sensor on the inside to effectively hit play/pause when you don the headphones. They also work as cellphone headset, and pause the music when you answer a call.

don't listen to him! work on a cure for cancer instead!

Woah, thats the equivalent of playing emo music for depressed teens!

I want shoes that will set the tempo of the song I'm listening to based on how fast or slow I'm walking.

Yes!

... though I imagine you wouldn't need shoes. The iPod could do it by itself, in the same way a pedometer works.

Would have to have some good software to determine beats. Easy with any sort of electronic/rock, hard with classical.

should they play the song backwards if you're walking backwards?

My video iPod automatically performs pause/resume on headphone removal/insertion very reliably when using iPod headphones but almost never when using third-party headphones (in my case, Sony MDR-EX71 in-ear headphones).

Of course this requires you to physically remove the headphones from the device.

Our new car headphones for the DVD turn off automatically when you take them off. The ear pieces are on a type of spring loaded mechanism, to put them on they swivel out, and turn on. When they are removed they swivel back in, and turn off. If you had them hooked up with some type of radio transmitter back to the device that controlled it, it would be great. Don't ask me how to make it work though. I will stick with woodworking.

I have the hardware for a generic USB interface, put that and a hub and the a USB headset together and it would be all software work for you.

you could use a mercury switch to close the "pause" circuit when the headphones were horizontal, provided you don't do much computing laying down.