The iphone and android support class compliant audio interfaces (via the camera connection kit on the iPhone) – the iPhone has quite specific power consumption requirements though – something like an Apogee One USB audio interface would give the best possible quality audio output from a smart phone on the move –
If you really wanted to go down the audio quality route, then you could use an audio specific kernel to replace the standard linux one in Android which will give you less jitter on the output (the difference is likely to be inaudible on the bus, through the vast majority of headphones).
Broadly, the DA conversion is passable in most modern gear, and it’s not worth worrying too much about the difference at that point in the signal chain – the quality of the source files (high bit rate MP3’s are an effective compromise) and the quality of the headphones are much more important.