I got a Toshiba Mini NB205 netbook with a 10" screen and a good 6 hours of battery life. Which is bigger than those portable DVD players. I didn't want to carry around an external dvd drive. I know itunes would rent and play movies, but I wanted a little more variety and easier searching.
I found Amazon's Unbox player. Good luck finding it, but it is here. You need a Windows machine. It looks fantastic on this netbook with the bright screen.
So the day before a long airplane trip, I pick a movie, let it download for an hour and I have 7 days to view it (after starting the movie for the first time). Works very well. Pretty painless.