The audio jack is actually a 4-pin connector. The additional pin is for the mic input.

1 Tip Left audio

2 Ring Right audio

3 Ring Common/Ground

4 Sleeve Microphone

Since the iPods have some 60mw output (30mw per channel) I would put the iPhone into that range.

Here is some other audio data comparisons:

Frequency Response iPad2; iPhone4; iPod classic 6th;

(from 40 Hz to 15 kHz), dB: +0.04, -0.55 ; +0.02, -0.16; +0.03, -0.18;

Noise level, dB (A): -90.3; -90.2 ; -90.2;

Dynamic range, dB (A): 90.2; 90.2; 90.2;

THD, %: 0.0045; 0.0068; 0.0049;

IMD + Noise, %: 0.0098; 0.012; 0.010;

Stereo crosstalk, dB: -49.2; -72.4; -67.5;

Source.