We ran across a comment on our review of the Jabra Cruiser 2 Bluetooth Speakerphone that sparked our curiosity. WPCentral member Score wrote,

"Cool product, but not really needed with WP7, if you have an aux outlet in your car. When the WP7 is hooked up to the aux outlet the phone uses your car stereo as a speaker phone."

More cars are being fitted with an auxilliary 3.5mm jack for MP3 players and it makes sense that the same jack could be used with the 3.5mm headphone jack on your Windows Phone. All you need is a male to male cable and a car cradle for your Windows Phone is a good idea as well (helps keep your phone from bouncing around).

In testing this set-up out with the Samsung Focus, call volume came in a little on the weak side (easily adjusted by cranking up the stereo volume) but surprisingly the microphone picked up my voice really well with the phone cradled about three feet away.  At highway speeds the microphone struggles a little to overcome the car noises but still call quality wasn't too shabby.  If you like driving around with the windows down, I don't see this as a viable option.

This set-up requires a few more buttons to push for answering calls and you don't have the noise filtering advantages the current crop of Bluetooth devices have. Still, if you have an AUX jack in your car, this is an inexpensive hands-free solution that works fairly decent.