I can't believe that those little external power supplies are all that accurate from a voltage standpoint, and with the cost of the type of TV that uses them (less than a few hundred dollars) the risk is not all that great, at least compared to the risk that my GPS runs when I start my engine.
There have been early postings from others (including MaineSail) who have done just what JimsCal suggested - wired the TV to 12 Volt.
I agree with the second part of SailDog's answer - no TV at all is nice. We compromise and carry one of the kids notebook computers on board. If I get serious about that, I'll order a car adapter for it so we can recharge it when the engine is on.
BTW, when looking at this in the past, the Target sold Polaroid 1513-TDXB and 1913-TDXB were identified as having external 12v power supplies. Since the last time I looked they have come out with a new TV which is specifically touted for AC/DC Compatibility, and that is the FXM-1511C. (not that it seems to be widely sold) It has a 12v DC in jack and a cigarette lighter adapter with some sort box in line, which I assume has some voltage stablization. They do however say:
"Do not connect the unit to the power outlet of the car until the car is running."
These are all TV/DVD combos
Here's their TV comparison page: Polaroid Portable DVD Player Comparison Tool @ Polaroid.com
I happened to have it in my history, I can't see to navigate to it.
If you are really worried, Radio Shack sells a 12v surge protector...