With a 1st generation iPhone, your best bet, for now, is to use the headset mic that came with it. When you plug that in, SoundMeter will use a default sensitivity that should get you pretty close to the mic's actual sensitivity.
Another option would be to make your own little mic, using an electret condenser microphone (ECM) capsule. Since the iPhone's headset connector provides a bias voltage, you shouldn't need to do much more than wire up the capsule to the headset mic input. You'll need to calibrate it, though.