I've been using this device from Brookstone as a hands-free system and it works very well with Treo.

http://www.brookstone.com/shop/produ...rds=hands-free

A cool thing about it is that is has an output at the speaker into which you can plug a cable to feed your car stereo. I use this output to feed the auxiliary input of another awesome product from a company that supports the Treo. It is called the MuzicVault.

http://www.mptronix.com/

Together I can manage 48GB of music and hands-free in the same system with only two cables going to the Treo. This setup can even charge the Treo as well.

The mptronix guys have been very receptive of suggestions and even implemented an auxiliary input for my Garmin iQue3600 so that the voice prompts would also go through the system.

I use the Garmin mostly with the MuzicVault because of the navigation functions, but when I'm doing quick trips, etc. I use the Treo to control the MV.

Check them out.