I've been using a radio transmitter in my car to play music from my treo (streaming internet radio) to my car's audio system. Here is a good one from Sewell direct with 10% off for treocentral users. This one is cool because it can also play mp3's off of a thumbdrive.

Go to www.sewelldirect.com
click on "promotions" (top right)
use promotion code "TREO_CENTRAL"
Go here: http://sewelldirect.com/thumbdrivedj.asp
You should see the adjusted price

Saves you a couple bucks at least!

(You will need to provide your own 1/8" to 2.5mm cable)