In previous versions of iPhoto, the user had to create a condition for each type of movie file that might possibly exist in the library (.mov, .avi, etc.).
The latest version of iPhoto can find all movie files for you, without having to add each extension, but the specific rule is not obvious.
When you create a new Smart Album the condition is actually found under the 'Photos' header. Chose Photos from the first drop down box and in the next two boxes you can choose 'is - Movie.'
iPhoto will gather together all of your movie files stored in iPhoto, regardless of their extension.
[crarko adds: I tested this, and it works as described. You can also do a search in the iPhoto Search box by Keyword = 'Movie' and that will find them although it doesn't create the Smart Album.]

