Does iphone support sms text via bluetooth in-car cradles ?

Hello - I've just bought my iphone 3g because Mercedes finally brought out a cradle for my car. Everything with the iphone has so far been great, and in the car, the phone and contacts work fine, BUT I there is no capability to view and send SMS Txt Msgs, and I don't know whether this is a Mercedes implementation problem, or an iphone limitation. With my old Nokia in its cradel int he same car, SMS txt was working fine, but with the iphone/cradle, the "TEXT" softkey on the Merc screen is gone.

The iphone cradle is a bluetooth cradle - I have been told that all the cradle really does is charge the phone and use the car's antenna, otherwise I may as well leave the iphone in my pocket - it is a bluetooth comms connection to the car, not a wired connection.

So, if anyone could confirm whether iphone does/doesn't handle SMS messaging over bluetooth car interfaces, that would be very helpful. Thanks.