I'm using my cell phone to hear podcasts, and I'm hearing it in my car using FM transmitter.
Unfortonately, the transmitter's jack is a standard 3.5mm, while cell phones (at least mine) use a 2.5mm jack.
There are cheap adaptors available for mobile mp3 players (also with 2.5mm), but the phone's socket is a bit different.
So I searched a little, and found out the SKN6183 adaptor. Since I couldn't find one in Israel I had to order it from EBay (cost me 1$ with 8$ shipment), but it was worth it.