I have been an owner for the Samsung Captivate (Galaxy S, Android based) for a while now. The audio quality for music seems pretty damn near the quality of my Zune HD Device. With good reason too, it is equipped with a nice DAC.
There have been many modders who have tweaked to chip to sound even better, a very good effort of the community. I for one have noticed a pretty decent change.
I consider the Zune HD to have a high quality audio experience. Music is crisp and clean, overall a very enjoyable experience.
The iPhone's DAC seems pretty decent from what I have heard, a very few select songs (one of my favorite to try is Billy Joel's We didn't start the fire), but definitely not as good as the Samsung Captivate.
What really gets me is the lack of quality on the Windows Mobile Phone devices, as well as every other Android device. What is up with this? Does no one care? I've tried the Samsung Focus (extensively) and the HTC Surround, and the audio seems kind washed out and not as sharp and clean. The Zune software on the device itself is beautiful and easy to us, but the audio is just not enjoyable. The audio quality is also lacking on HTC Android devices I've tried as well as the popular Motorola Droid devices.
Do people not care anymore? Certainly portable standalone players like the iPod and the Zune HD will HOPEFULLY continue to exist and provide excellent audio, but there is a push to use your phone for almost everything. A Zune Pass on the Windows Mobile 7 platform is quite nice. You have access to all of the music Zune has to offer. This is of course at the cost of audio quality.
Does anyone care? There are very few phone devices out there that offer anything decent. Certainly quality doesn't have everything to do with the DAC, but it is a big part of it. Pretty crappy status quo.