So I was writing on my thesis when I thought, what could be a nice way to control my arduino!
Having recently watched Dollhouse (Joss Wheadon wohoo!) the idea came to my mind to use a microphone on the arduino to decode an audio-track to serial data! As I recently saw, the Android platforms features a serial to Audio-convertor, which was originally intendet to be used with a modified audio-jack I think. But we could always use the speaker directly!
Some research gave me, the Arduino ADC has a maximum resolution of 9600Hz, which would not be enaugh for a baud rate of 9600 (start/stop bits) but should easily be enaugh for 4800 or 2400. So it should be possible to use a midi-creating-sound-program to send different commands to the arduino. It will most probably not sound nice, but could be the most cheap, widest available method for (one-way) wireless communication.
The great thing about this is, that almost any mp3-player can be used to issue commands!
So what do you think?