I didn't know anyone could get text messages on the iDrive screen. They can already do it in Europe. It hasn't come to the US yet due to safety reasons (read ambulance chasers). Apparently it is coming to the US by the end of 2010. You will only be able to read the text when parked for safety reasons so the system will actually read the messages to you.
It is demonstrated with a blackberry in the following clip but they state it can be used with a phone that supports the latest MAP-bluetooth protocol. Maybe the iPhone 3G doesn't support the latest MAP-bluetooth protocol.
http://www.youtube.com/watch?v=4B_I4P1R2x8