In my experience with a car radio, it's measured in cm. Perhaps indoors with a fixed antenna aimed at the N900, across the room.

Updates to the fm transmitter broke the RDS so it always just shows "NOKIA" which is useless IMHO. (I guess there's a radio somewhere that shows the "info" field, but I've never seen it). So every radio station I've ever seen just rotates all relevant text thru the station name field. But you can override that with a direct call to fmtx.