I got interested in this after I saw Square use the headphone jack on the iPhone to send credit card data.
What's the average bandwidth of the headphone jack on the iPhone, average notebook, and average mobile device?
Can it be doubled by sending different data streams on the different channels (left/right)?
One issue is the bandwidth of audio cables, which I won't go into here. As for audio ports, assume a soundcards with a maximum sample rate of 44,100 or 48,000 samples/s at 16 bits/sample/channel, resulting in a maximum bandwidth of 22.05 or 24 kHz (basically a result of the Nyquist-Shannon sampling theorem, though for sound sampling, the sampled signal would also have to be continuous-amplitude for this theorem to apply) and a transfer rate of 176.4 or 192 kBps for stereo.
According to Studio Six Digital, the line-in on the iPhone supports a max sample rate of 48 kHz. The mic on the 3G version also runs at 48 kHz, while the 1st gen iPhone's mic sampled at 8kHz. I haven't been able to find bit depth specs for the iPhone, but I believe it uses 16 bit samples. 24 bit samples is the other possibility.
According to Fortuny over at the Apple forums, who was quoting an Apple Audio Developer Note, the line-in on a MacBook support up to 24 bit samples with a 96 kHz sample rate, for a data rate of 576 kBps. Apple's MacBook External Ports and Connector's page lists the max sample rate as 192 kHz, but they may have switched that with the max sample rate for digital audio using the optical port.
For a rate comparison, phone systems had a sample rate of 8 kHz at 8 bits/sample mono, resulting in a max data rate of 8 kbps. FM has a sample rate of 22.05 kHz at 16 bits/sample/channel and is stereo, resulting in a data rate of 88.2 kBps.
Of course, the above calculations ignore the problem of synchronizing the data stream and error detection and correction, all of which will consume a portion of the signal.