Get a Samsung P2 or P3 or M1. Great media players. Then get a cheap phone that has bluetooth. Sync the two, and you'll be able to keep the phone in your pocket or backpack and make/receive calls from the Samsung.
I personally am not a fan of apple products. But the soundquality of the iphone is well known, and can't be denied. And apparently the new Droid is also a very capable mp3 player. You can of course carry a dedicated mp3 player with IEMs + microphone synced by bluetooth to the phone...but it's not really necessary. Why carry an extra load when you don't have to?
Now, if you plan on spending many hours listening to your music nonstop, and don't charge often, then the above becomes a better idea since it won't drain your phone battery so quickly. So think about your typical listening habits and whether you have access to a charger (and the will to carry a charger depending on the situation) and choose accordingly.
I've used both my iPhone (first gen, before I lost it) and my Blackberry Bold. I would recommend the iPhone and not the Blackberry. I got GREAT battery life on my iPhone (approx 30 hours playtime I think) and really only charged it about once a week. The iPod software was really nice, as long as you don't mind syncing with iTunes. The sound was pretty good too, imho - though apparently the new versions are much better.
The Blackberry was another story. The sound quality was OK, though it suffered from pretty severe bass roll off. The media player software is pretty subpar, and eats the battery like crazy. I can just barely get through the day listening to music at work (around 6 hours), a phone call or two and occasionally surfing the web on break and have 10-15% left on the battery at the end of the day. On the plus side, I'm not tied to iTunes and can drag and drop from any computer/operating system.
I'm thinking about getting the new iPhone to replace my Blackberry and Cowon S9. I'm not thrilled about being tied to the Apple ecosystem, but as an all in one solution the iPhone seems tough to beat.
The iPhone is one of the best, if not the best at giving you great SQ. I have the Bold 2, and it hardly drains battery life, (the above guy talks about the Bold 9000) but its SQ is OK, maybe slightly above average. The iPhone is the best though, if you must use a phone only for your music needs.
I'm really disappointed with headphone out of my G3. I notice that I cannot hear as much details as my x1051. On my x1051 I can hear Glen Gould humming on the background as he is playing piano, but on the G3 his humming is too faint? Is the signal washed out on the poor signal to noise ratio of the G3? I'm very disappointed. Line out is another matter, I'm not sure if it actually has a good line out because I get low frequency distortion on normal EQ for a bass heavy mp3 file that I don't get with other mp3 players.
I am using this phone right now and have used it for a while.
The sound is espicially noticeably bad when u turn it up and it gets all muddy.
Thats why i recently switched to the Sansa Clip + cuz I couldn't find a decent mp3 that was cheap.