From Wired START:
Sure, you could fill your video iPod with episodes of Knight Rider from iTunes for $2 a pop. Or you could save some cash – and your soul – and fill it with files you convert from your DVD collection using freely available software. Problem is, you’d have to break the law. Though it’s theoretically OK to back up DVDs for personal use, it’s illegal to override commercial copy protection, a necessary step in the process. Proceed at your own risk.