If you've been looking for a convenient portable device, you might be interested in the MAGNUM released by YUBZ today. This player performs many functions at once, including music player and mobile speaker.
Equipped with Bluetooth, you can listen to music over A2DP and your listening will be interrupted to take calls automatically. All you have to do is press the power button to "answer" a call. Once you do that, the music player turns into a speakerphone.
Once you end the call (by pressing the power button again) the device turns back into a music player. It uses four AA batteries or can be powered through AC. With a 10-hour battery life, you're sure to have plenty of play time. You can get the MAGNUM now from YUBZ for $130.