Wouldn't one that comes down from the roof be useless for the driver? Like sharp1183 said, Clarion makes some awesome in-dash DVD players. Check this new one out:
Clarion VRX755VD (even has IPOD integration).
Clarion will begin sales of the "iPod-compatible DVD/Wide 7" AV center unit", a car AV unit that supports control of Apple's HDD audio player via a touch-screen LCD, in North America from January 2005.
The "AV center unit" connects to the iPod via a special cable and displays the iPod LCD contents and control keys on its 7" LCD screen. The iPod is controlled via the AV center's LCD touch-screen display. The unit is also equipped with power supply and charging functions for the iPod.
Beginning with sales in North America, Clarion plans to gradually introduce more car audio and navigation products that support the iPod to places such as Japan and Europe. Clarion also plans to supply these products as OEM equipment to car manufacturers.
According to Apple's director of iPod Product Marketing, Stan Ng, "the car is one of the favourite places to listen to music and Clarion's innovative solutions allow music lovers another option for easily listening to iPod in their car." The iPod-compatible "AV center unit" will be displayed at the "2005 International CES" in Las Vegas.