In addition to the three films mentioned in the write up, I would humbly suggest the following movies deserve inclusion:

1. Oklahoma -- Sort of encapsulates what a big screen musical of the 50s is all about.
   
2. The Great Escape -- Great cast and gripping story.  One of my favorite WWII movies.
   
3. Dirty Harry -- If for no other reason than to recognize one of the iconic figures of American movies.
   
4. The Sixth Sense --  The mother of all twist ending movies.
   
5. Die Hard -- I mean if you're going to have Rocky on the list, why not this one that spawned another franchise.  Plus, it's a great movie.
   
6. Airplane -- What's not culturally or aesthetically significant about Lloyd Bridges declaring "I sure picked a bad day to stop sniffing glue" and Robert Stack parodying himself?
   
7. The Omen -- A really, really great "the Anti-Christ is among us" flick.
   
8. Reefer Madness -- If this isn't culturally, historically and aesthetically significant, what is?

Shoot me, but I'd add Godfather III to the list to round out the saga of the Corleone family.  I know this will never happen in my lifetime, but if there isn't at least one pron movie on it (think Deep Throat) this list fails to capture a good portion of the history, culture, and aesthetics of that last half of 20th century American cinema.

Really, the films that most need preserving are the ones that nobody cares about. Considering how easy it is to milk money out of the nostalgia industry, and how cheap it is to press DVDs (or BluRay/HD-DVD for that matter) these days, most films with any sort of audience appeal should be reasonably safe for years to come. Therefore, I do think it makes sense to concentrate on the older and more obscure films and hold off on the more recent works. Time and perspective also help in determining whether a film really deserves to be there or not - sometimes it takes a few decades to realize the impact a certain film might have had, and sometimes films that appear to be hot stuff end up being somewhat underwhelming in retrospect (Forrest Gump, for example, or really pretty much any Tom Hanks drama, for that matter).