If you can honestly justify spending too much money on an iPod to watch a video on the train then i feel sorry for you.
Don't get me wrong i'm not anti the whole concept, i'd use one if they were free, but for however overpriced they are i can't see what drives people to buy one, you don't need to watch a video at every spare second. You can be just as easily entertained, for free, by looking at the different kinds of characters on the train.
Or by laughing at foriegners that lock themselves in the toilet on the train and break the door open. See my avatar.