This is probably not advertised as much, but the battery life of a portable DVD player sucks. So much as you'll get two hours battery life before the thing dies. So on a plane for example you get to watch one movie and then thats it. (Unless you have another battery). [Kind of reminds me of the Gamegear]
For what its worth cheap small laptops with DVD drives are coming down in price all the time, and especially on long flights you have the right power connections this is defintely worth a look at. Plus, you get a million miles more functionality...
Liksang sell portable VCD, MP3 units not much bigger than a discman. Also play JPeg files as well (so if you have a digital camera it can act as a mobile home projection unit). Hope this helps everybody going down this road...