here is some scientific testing of the frequency response of the HTC Droid Incredible's 3.5mm headphone jack for anybody who is interested. this is pink noise played out of the headphone jack and input into my laptops microphone jack. i calibrated the sound card on my laptop so these results should be accurate.
pretty flat. the top end has a slight decline, but it's not a sharp drop by any means.
for anybody that has never done any measuring of audio with a pink noise track it will look jagged. you are supposed to take several measurements and average them together. this is just one quick measurment, so it will look jagged.
this is of course only 1 factor that determines sound quality. this is just frequency response. it is relatively flat which is good.