As of June 2010, Apple's Terms and Conditions indicate that television shows and movies purchased from the iTunes Store are "protected," and as a result you may only play them using an "associated device," which means a copy of iTunes on an authorized computer or another approved device (such as an Apple TV or iPhone).
The FairPlay copy protection used on purchased iTunes video files is known as Digital Rights Management (DRM). Because you agreed to the Terms and Conditions when you made the purchase, you cannot legally circumvent the DRM on the video files. Keep in mind, however, that the Wikipedia article describing FairPlay lists several options for circumventing FairPlay.
Best of luck.