I have to say, I agree with the person that said you should get an ipod and be done with it. I bought one of those LG Muziq phones from Sprint, thinking I'd be able to drop carrying the ipod around. Unfortunately, I found that any phone calls interrupted the music and if you are taking calls, you'd have to unplug/plug the device while driving in order to take the call. I don't know the details of the phone you might get but in general, I was underwhelmed with the end result. ( I was using FM transmitter, but same idea) Not to mention the sprint music player is absolutely shitty. I've heard the verizon software is better, but if the chocolate is anything like the muziq, then I wouldn't even consider it as an option. Also, that muziq phone only worked with up to 4gb micro sd cards, supposedly. 4 gigs didn't hold enough music to make me want to switch from my 30 gb ipod.
Honestly, the best solution IMO is either an ipod with direct control from the head unit (which you probably don't have if you are using 3.5 mm jack) or a USB equipped head unit. In either case, you control both devices from the head unit. Not to mention with the USB, you can cheaply get large capacity drives (16 gb and up). Also, if you do have an ipod ready head unit (i.e. dock connector cable & digital signal) then you have the added convenience of the unit being charged by the head unit whereas otherwise, you'd have to deal with both changing the songs on the ipod as well as the fact that it would need to be charged.
The ipod/head unit combo is amazing. Its just like having a hard drive hooked up to your stereo (duh). I just throw it in the glove box and switch albums on the head unit, and its fully charged when I remove it.
I'm sure none of these solutions are applicable to you, but since I just had a similar scenario within the last month, I thought I'd share my experience.