Are there any car stereos out there that support AirPlay?
AirPlay works over UDP and it requires that both the sender and receiver device exist on the same TCP/IP-based network. That's generally not possible in your car because you aren't running an 802.11-based wireless network in that space.

For wireless audio streaming in your automobile you want to look for a head unit that  supports the Advanced Audio Distribution Profile (A2DP). This is a BlueTooth spec that lets two BT-enabled devices exchange high quality audio streams. Your iDevice supports it, you just need an A2DP compatible head unit and you can stream your music to your car system without any wires.

Happily, there are many head units that support A2DP on the market today, both aftermarket and OEM.