Well...

A Centrino/Pentium M based laptop will generally give you at least three hours of battery life. They also can be found in smaller form factors, with 12" (widecreen or 4:3) LCDs which makes them generally usable in Economy (the widescreens look better for DVDs and are not as tall).

DVD players, especially the 7" widescreens, are pretty darn cheap. When you get into the 10" - and especially the 12" - your in PC price territory, so you might as well get the laptop. Most also last around 3 hours on batteries.

One thing to note about DVD players is where you can get spare batteries and how expensive they are. A DVD player that does not offer spare batteries won't help you for hours 4-18.