Thinking better of it, the theme from Titanic (My heart will go on) is one of the movie themes which can be played in front of an audience because actually it is a song. Other movie themes (like Star Wars, Dances with the wolves, Jaws, etc) are nothing more than musical phrases that last a few bars and serve just the purpose of conveying a state of mind, an emotion, etc.
The examples made by DonM (Blues Brothers 1 & 2 and Your cheating heart) are actually soundtracks made by different songs (like in American Graffiti 1 & 2).
So I think that there is a difference between composing music for a film and putting together different songs to have a soundtrack (just like when an amateur makes a video of his holidays and has to choose the songs to go with the scenes).
The difference, of course, is evident, when you try to play that music, even for yourself. I like some movie themes (like The Good, the Bad and the Ugly by Ennio Morricone), but playing them on my keyboard and being satisfied with the result is an entirely different thing.
Mmmhhh... what do you think?