The Treo bluetooth handsfree profile is broken and useless on it's own. You need to add the JetWare bluetooth extension to get it to work:
http://www.jetwaremobile.com/
I've tracked it down a little deeper and found that subtitles do turn off as they're supposed to, they just default to "on" every time a movie is played.
So I guess what I'm trying to do is change the default to "off" which would be the normal behavior for every DVD player I've ever seen before.
For whatever reason, the AVIC-D3 won't stop displaying subtitles on DVD movies. I've got nothing against subtitles, but the irony of having to read a movie screen in a car is overwhelming.
Any ideas?