Many people have add-on FM transmitters for their in-car audio, because all cars have FM radios these days, and it's a good stop-gap measure for connecting your portable media player and your car stereo. I don't think we will ever see digital radio (DAB+) transmitters for the same purpose, though. Car stereos with digital radio receivers are much more likely to have an auxiliary input, meaning you can connect any audio device directly with a simple audio cable.
Mokalus of Borg
PS - Or Bluetooth for a different kind of wireless connection.
PPS - We're actually in kind of a weird transitional time, I guess.