Im lookin for the same thing. I can stream music an audio from films thru the car speakers fine but calls dont seem to work which is odd and daft.
Guess we will have to wait for an app to allow this or they may add it in the next firmware PR1.2 which is due out next month, so they say? :-P

hmm..... is there someway now to stream calls through fm transmitter??

I would also like to be able to do this. The speaker phone is way to quite for me to hear over the road/wind noise in my car. It's totally worthless without using the ear buds or another hands free ear piece.

Just keeping this thread alive as I too would like (love) this feature. I now have a nice mount for my n900, but built in speakers are too quiet for calls while driving (speakerphone).

Could someone consider an App that allows for this? Or perhaps Pr1.3 will allow it ;-)

FYI, there is a thread for this already here

And a bug filed here

Please report this in the bug and increase the vote count for getting this fixed in next release.