I've been looking for a cellphone that can play music as well as a high-end, dedicated digital audio player can, but so far I've been disappointed (the Nokia N91, which apparently offers drag-and-drop support for the playback of up to 3,000 songs and allows you to use your favorite headphones, could be the next one I'll check out).
At CES, a company called OpenMoko will unveil what they call the world's first open, Linux-based cellphone. I feel like they could encounter some difficulty getting carriers to support this thing, since the fact that you'll be able to install all sorts of third-party software could result in music sharing, DIY ringtones, and other weird apps that exist outside of the music store infrastructure. The limited specs don't mention music mention a standard 2.5mm audio jack, so the phone will almost certainly be capable of playing music (clarification: you would need a minijack-to-microplug adapter).
I'll take a look at the OpenMoko phone in person at CES next month, and will hopefully let you know more how it works from a music standpoint then.