Arduino Frequency measurement

I've been thinking about ways audio output from an iPhone could communicate with an Arduino.

Oh yes, you could use DTMF with an external DTMF detector chip, but what about doing frequency detection without any active external devices?

Here is a Frequency Measurement Library for Arduino w. Atmega168/328 that needs some external resistors and capacitors, no active devices needed. A 1:1 audio transformer might be wise as well.

Supposedly this works up to 20 kHz!