Thought I'd chime in here.

Volvo S40, N900 with the latest available firmware. Reception is excellent, I even get 'Nokia' on my radio (which was a nice surprise).

In fact, its so good I keep the device in the center console armrest.

I didn't notice any difference with the screen being open or closed.

Frequency: 90.7

But on a related note, is there some app or widget to toggle the radio from the home screen? Its really a pain to go to settings > fm transmitter > on every time :/