How to interface a Blackberry Torch with a serial device?
For under $50, you could buy an SPP bluetooth module (and a microcontroller dev board if you need one)

For $50 you can buy a complete WiFi module with a serial interface.

If you use the headphone jack, investigate Frequency Shift Keying. FSK is a modulation scheme used in audio modems.

Here's a couple of relevant projects for pointers:


http://sree.cc/electronics/arduino/arduino-as-an-fsk-modem
http://code.google.com/p/arduino-iphone-shield/


On the Blackberry side, you could either do the same thing, or play back pre-canned .wav files.