I use Rockbox 3.6 on my Sansa e280 and I love it. Many, many thanks to the developers.
I use my Sansa in many places that I do not want a phone. For example, when I am at the gym, not only do I not want my phone going off but my phone is too big and heavy for me on a treadmill. You might think the size difference is minimal but I doubt you would agree after running a marathon.
My phone is probably going to be a somewhat expensive device for the foreseeable future but I got the Sansa refurbished for $30. Why would I risk dropping my iPhone into a river on an wilderness hike when I am out of cell range anyway?
I often listen to my Sansa as I fall asleep and sometimes I crash wearing it. I wrecked two Blackberries (cracked screens, gimped jacks) doing that before I got the Sansa. The smaller Sansa seems much stronger. The iPhone 4 may be built to be strong but all that glass still has to be a liability. Music players do not need giant screens.
It is illegal to use the phone while driving where I live. Do I want the music to stop when a call comes in?
Back in the bad old days of Palm Pilot mania I used to say that I would not be happy until it was all built into my phone. My phone was always with me everywhere that I needed a Palm Pilot so that seemed obvious. I still only want to bring one device in such settings, so I certainly want my phone to play music. That said, I hope the day never comes when I cannot buy a dedicated music player. I also hope that the Rockbox guys will pump out firmware for it because I love what they do.