I don't know if you guys knew this already or not, but I'm going to give it you anyway.

There's a way you can listen to mp3s and Listen to callers through your car's speakers. Go to RadioShack, and ask for a 274-373 for $3.99 and a 43-2003 for $6.89. You'll also need an FM modulator. RadioShack has one for $29.99 that can use a AAA battery or you can use the supplied cigarette lighter adapter to power it. You can also get one from Wal-Mart for $19.99 that only uses 2 AAA batteries.

Plug the extention cable into the headset jack on your Treo, and plug the adapter in to the other end of the cable. Then plug whatever FM modulator into the adapter. Just tune your car radio to one of the four stations on the modulator and you can listen to Shoutcast streams your mp3s in your card AND you can hear callers through your car's speakers. The callers can also hear you too. Just put the treo face down in your lap while talking, that seems to be the sweet spot for me in my car. You just can't turn you car's radio volume up too loud or the callers will hear themselves with an echo. This way is better than spending 50 or 60 bucks on something that does the same thing.