on long car trips, it seems like everyone in the car (except, me, the driver) has headphones on. no one will miss the lack of rear speakers.
If passengers want to DJ, you can get one of those little FM transmitter thingies that plugs into a phone/table headphone port.
edit to add: if Slate is successful, I wouldn't be surprised if a decently sized ecosystem pops up around easily installed custom sound systems and the tablets (possibly with headphone jacks!) to control them.
A lot of Bluetooth speakers today can fill a car with a sound wall better rear speakers used to. Apple says you just need two of their Bluetooth speakers to fill a room in a house with great stereo and reasonably good surround sound. The square footage of a car is generally smaller than the supported room size.