There are a couple issues why we can not assume goodness from every device with mp3 function. Phones and audio-only devices no longer have a big sound gap in quality.
This stuff is not analog hardware, so the handling of GAIN in the chip parameters is VERY important. Low gain, weak a$$ audio. High gain, distorted audio. The gain curve in relation to volume needs to be in check, else too low or high. Due to EU restrictions, Archos is an example of sucky audio fidelity from 3.5mm jack. The gain & volume curve is off, so the audio output is too low and mid range dominated.
Then you get into the dynamic of headphones and the ohms, since the safest bet is getting buds with 16 ohm and no more. I even notice an output hit on my Droid 3 when using my 18 ohm Klipsch buds. Not so bad on the iPod4, but it has higher output.
The other issue is how the audio is managed on the device and the actual circuit and line isolation. This part apparently is where the DX2 and Bionic got things wrong. Some report the DX2 is better now, but store demo after updates still had the issues.
Added: I am a little picky with audio, since pick up if the sound stage is wrong, or highs & lows out of place. Phones have come a loooong way! Droid 3 is a little low output, but sounds good

Sadly, the iPhone 4 & 4s have better output

I have an iPod4 though and just as good
The
Nokia N900 is probably the greatest sounding phone to date and same level as Cowan audio players. Shame that Maemo support by Nokia sucked. The N900 actually has a built-in, in line amp to help sound. How cool is that?!