I have a 2007 Prius with the Bluetooth phone option. If I was on the phone I could never answer another incoming call with my Blackberry Pearl. When I would try to answer it, I would disconnect the original call and was still unbale to pick up the second call. I just upgraded to the Blackberry Curve and the call waiting feature works as it should when I'm in the car and connected via the bluetooth
I have a Treo with the same situation. The only way I have ben able to answer a second call is to do it on the phone as if I was not using BT.