If your cell phone is capable of using the Bluetooth A2DP stereo profile and you purchase a hands-free Bluetooth speakerphone that comes with similar functionality, it may be possible to stream music from your music phone to the speaker.
Read our article on Cell Phone Headsets for more details, but if you want to have the best experience possible, you should consider a higher quality Bluetooth A2DP headset that will give you stereo sound and without wires!
Stereo Bluetooth A2DP: Seeing how the iPhone is supposed to replace both your mobile phone and your iPod, it's a little strange that Apple didn't include stereo Bluetooth in the first generation iPhone.
Although it is not described on the official Apple Web site, Apple CEO Steve Jobs did mention in a keynote speech than the Apple iPhone will come with the highly popular A2DP Bluetooth profile.
If your primary interest is in listening to music from your iPhone, then you may be more interested in getting a Bluetooth stereo headset (A2DP) as described above.