You probably would need headphones connected (fm radio sucks without them, even if you force-start the radio, so i'd imagine data mixed in would be that good either)
Also, does ovi maps support this? This is actually the big problem. Since that app is not open, this functionality would need to be implemented by nokia. And they can't even be bothered to get maps 3.0 on the n900 (or, for that matter, make an official fm radio app)