While downloading movies from the iTunes Store is nice, they don't have much to offer. If you want a favorite DVD on your iPhone, is the best option.

HandBrake is an open-source application for Mac, Linux, and Windows that converts DVDs into a file your iPod/iPhone can read.

Using HandBrake couldn't be simpler. Pop in your DVD, choose the iPhone preset, and let it go. One of the downsides is that it can take a painfully long time-- I would suggest letting it work overnight if you're importing a feature film.

That aside, HandBrake is the easiest way to get your movies onto your iPhone.