I upgraded to the iPhone 3GS about a month ago. I thought the sound quality was better (no hiss on my Shure SE 530 IEMs, more bass on my Sennheiser HD 650 cans), and today I put it to the test. I compared the iphone headphone out to my Pico DAC. I listened through my Sennheiser HD 650's. I played lossless files on both devices, and I tried it with two songs: Radio Attack, by Prefuse 73, and I Am Trying To Break Your Heart, by Wilco. Both songs are sonically complex, and are considered to be meticulously engineered. I prefer a testing modality where you switch back and forth between both songs every few seconds (you need an extension cable for the iphone or else it pauses when the headphones are unplugged) rather than comparing impressions across an extended listening session.
Compared to my "gold standard" Pico DAC, the iPhone 3G S was indistinguishable on both songs. The midrange details, the bass thump, were all exactly the same, to my ears. The maximum volume the iPhone could put out was a hair louder than the Pico at max. Details, separation, and the sense of space, were all the same to me, as far as I understand those concepts.
This is not a knock on the Pico, by any stretch: the Pico is a good improvement over the native soundcard on my laptop. Rather, it's a celebration of the iPhone 3G S. Since the beginning a nifty little device, to be sure, and now a legitimate source for audiophile listening. I imagine it is new hardware, rather than the 3.0 software, that is making the difference, but I wish I still had my 1st gen for the comparison. I have an ALO cable coming to me that will allow me to compare the iphone's line out + pico amp to the headphone out. If people are interested, I can post that outcome once I know it.
Note that this was not a blind test. But, my decision went against my bias: I *wanted* to prefer my lovely Pico over some consumer-electronics fancy toy. My hearing is within normal ranges. I've been a music listener for years, but I am not a performer - there are others out there, surely, who will be able to recognize a difference, but I for one cannot.
Have any others done direct comparisons between their 3G S (or earlier iPhones) and their portable rigs?