The writer of eHphone.ca has prepared this video presentation on how to add movies to your iPhone 3G.
If you are taking a plane trip soon, you may need to know this before your trip to possibly get few movies with you on your iPhone 3G if you have a long flight.
Just remember you need movie files to add movies to your iPhone 3g. You need a movie file. The most common types are ISO, IMG, AVI (divx or xvid).