The headset input device (not available on original iPod touch devices) supports units of FS (full-scale, or maximum input before clipping), and V (volts). The single headset input channel supports various engineering units, in addition to FS. One reason for including separate units and sensitivities, for the headset input device and the headset input channel, is to enable a voltage calibration, which then allows you to enter a standard sensitivity for a transducer (e.g. 50 mV/Pa for a microphone or 100 mV/g for an accelerometer).
The default voltage sensitivity of the headset input devices is 15.5 mV/FS and the default pressure sensitivity of the headset input channel is 7.0 mV/Pa, which should get most iPhone or 2G iPod touch devices within 1 or 2 dB of the actual sound level when using an Apple-branded headset microphone. The accuracy of the measurement does depend on the nature of the acoustic signal, however. For example, an acoustic signal that is dominated by low-frequency energy (below 100 Hz) will appear to have a lower sound level when measured by a microphone connected to the iPhone 3G's headset input (see above).