Not really. Could you carefully record something on an iPhone and have NBC broadcast it on the evening news? Yes, but it's not the way to go for good portable audio recording. Is the camera on the iPhone good enough for a professional photographer? Should that photographer sell their other cameras and only use the iPhone?
I would use a good portable recorder. Many people use the Zoom H2 because it's pretty decent, and it's also one of the cheapest options. I'm looking into buying an Edirol EDR09HR for myself.
ALL telephones are primarily designed to be efficient at picking up the frequencies of a human voice at VERY CLOSE RANGE. This make sense when you understand how they're commonly used.
So there's no reason to make the microphones particularly sensitive or (and this is most critical in the area of recording ambient sound with them) to build in high-quality low-noise circuitry into them such that they achieve a robust signal to noise ratio in typical recording.
They're designed to be PHONES. Not field recording devices.