Portable DVD players will be for people who aren't tech savvy.
The minute I got my first iPhone (a 3G), I learned how to convert video files to iPhone format, load them and then watched them on the iPhone when traveling. I have an iPhone 4 and the 32GB are great for music and video. Last cross country trip I caught up on all the Modern Family episodes I had missed.
Free format converters like Format Factory are very easy to use and have a list of mobile devices and convert to that format when you select it. Just make sure to unclick the Ask.com toolbar when installing!