http://itunes.apple.com/WebObjects/MZSt ... 67071&mt=8
CV Generator is an iPhone app that generates audio signals which may be used in conjunction with suitable hardware to control analogue synthesisers via Control Voltages (CVs).
The app uses the same technique as the Silent Way AC Encoder plug-in. You therefore need hardware such as that described on the AC Encoder page to turn the iPhone's audio output into CVs.
It is recommended to use an iPhone dock's line out rather than the iPhone's own headphone output, in order to get a stronger signal to begin with. Even then, the dock's output is quite low compared to, say, a proper computer audio interface, so you will almost certainly need an active circuit with reasonable gain to get a usable CV range.
CV Generator can generate two CVs, from the left and right channels of the iPhone's stereo output. The current version of the app has two modes for controlling the CVs: an XY controller for controlling both CVs from a single touch, and a pair of sliders for controlling the CVs independently.