By default, movies and TV shows in the iPod portion of the iPhone and iPod touch play in landscape view. The biggest change with respect to video orientation came about with iPhone OS 2.0, which provided the option for landscape-right viewing (that is, with the volume and silent/vibrate buttons facing up). While most videos could, and should, be viewed in landscape, there are times that call for a portrait perspective -- for example, when placing your iPhone into a dock, which is what I do when working out.
Getting your iPhone to play a movie or TV show in portrait mode requires a simple change in the video's tag, from TV Show or Movie to Podcast. Simply right-click on the file and select "get info "(or you could use the Command-I shortcut). Then, in the "Media Kind" section within the "Options" tab of the video, select Podcast. Now, the videos will show up in both the Podcasts and Videos section of the iPod portion on your iPhone.
Obviously, this isn't something you'd want to do with every movie or TV show in your library. So, to revert the file back to its old self, you'd want to follow the same process and change the video back to a TV show or movie.