I just stumbled on this:http://itunes.apple.com/us/app/thermometer-measure-temperature/id445589998?mt=8
I think the arduino side is similar to this:
I didn't find the sketch that is needed to interface with that iphone app.
Sam had mentioned this type of approach when we were thinking about arduinos and browsers before. However it isn't currently possible to access the microphone from a browser.
Putting that aside though, it seems like it would be possible to make a very cheap temperature sensor which used the headphone signal as an input and sent the result to the microphone. It wouldn't require a battery or any digital processing, just a thermistor with other analog components which would modify and return the input wave. Since the headphone is stereo the second input might be useful for calibrating especially if the modification by temperature is amplitude based.
I'm surprised this hasn't been tried, seems like it could made for less than a dollars worth of parts. It might take some experimentation to find the cheapest analog circuit that would modify the input in a easy to detect way.