So far the Treo 650 is having many problems with Bluetooth - check out any of the Treo user groups.
Lexus is only certifying a select few phones to work with the Lexus implementation of Bluetooth - and so far that doesn't include the Treo. For example, with Sprint there is only 1 phone nationally that is certified - the Sony T608. Try http://lexus.letstalk.com for more information on Lexus certified Bluetooth phones.
I have an LS430 and a Treo 650 and it doesn't work so far. Lexus customer service said that if it isn't on Letstalk then they can't help.