JVC just announced the September release of the KV-DV50, an in-car DVD player that's also able to play discs filled with Divx videos, MP3s and WMAs. It can be plugged into the cigarette lighter socket, so actually integrating it into a car's AV system shouldn't be too difficult. In addition to playing back store-bought DVDs, the KV-DV50 will also play back all the common burnable DVD formats, including DVD+R/RW and DVD-R/RW; dual layer DVDs don't appear to be supported, however.
JVC's KV-DV50 is expected to be release in Japan this September for around $250. North American price and release date information are still unknown.