Arduino wireless communication
You could transfer the data over the microphone part of the microphone/headphone port present in almost all of these phones. Project HiJack provides an introduction to these methods. Most of these solutions transfer the data with Bell 202 modulation. There’s even a $79 development kit available.

It doesn’t sound like you need to actually power your equipment through the headphone jack.

You may also be familiar with the Square card reader. The older versions were passive and simply hooked up a cassette tape head to the microphone connection through a resistor, i.e. analog transfer. I believe the newer versions have a battery and a microcontroller and may also use Bell 202 or similar modulation.