"Narnia" refers to the entire 8 book series.
In the context of the December 2005 movie season, it's unambiguous. They're marketing the movie as "Narnia" with TLTW&TW as a subtitle.
(there are 8 books? I thought there were 7.)
I will probably see "Brokeback" sometime in the future, if I can take it out for free from my library. Just to see what all the fuss is about.