from apples FAQ page
"Siri also works with headphones and Bluetooth headsets. When you’re using headphones with a remote and microphone, you can press and hold the center button to talk to Siri. With a Bluetooth headset, press and hold the call button to bring up Siri."
so i imagine if you use the "hands free control tweak" you can activate siri, then you can have siri send a text message.
that might be kinda a roundabout way of doing this, but its a start... and i think there is a tweak or even just a system option that will read your texts to you when you receive them.