The US has launched about five enduring art forms–comics, tap dancing, jazz, banjo and the musical comedy. Of these, I’d say the musical comedy (which, after numerous false starts, began in earnest with Oklahoma!) is the most important. So my vote is for opening night on Broadway of Oklahoma! March 31, 1943 at the St. James Theatre. (It had been beta-tested in New Haven before that, but Broadway is the opening that counts.)
If you go to Japan, they’ll tell you thay Jazz is king.
As well they should. 
I think Diaghilev’s ballet set to Le Sacre du Printemps by Stravinsky might be a contender, if nothing else because of the audience reactions and the audacity of the music:
http://www.straightdope.com/mailbag/mriots.html
Another vote for the career of Thomas Edison. He gave us the mediums which redefined popular culture forever.
Artistically? That’s a hard one. I don’t know if it can be distilled to a single event but one seminal “moment” that comes to mind would be Bill Haley recording “Rock Around the Clock.” Another (as emotionally ambivalent as I am about this) would be D.W. Griffith’s *Birth of a Nation."
Also Mark Twain needs to be somewhere on the list.
Just among musical genres, you seem to be forgetting rock and roll, country, bluegrass, and hip-hop.
Rock and Roll: Blues dumbed down so white people can play it. Lots of toe-tappin’ favorites, but to date, no masterpieces.
Country, bluegrass: Strongly influenced by rural British Isles music. Remember the band in Lair of the White Worm? Yeah, guys like that predate Bill Monroe and the Carter Family.
Hip Hop: At least a Puerto Rican co-creation, so it would have an asterisk by it. Cultural popularity is undeniable, but cultural importance is yet to be determined. Again, no real masterpieces yet. I’m sure the counterexamples you’re ready to cite sold a bazillion units, though.
No, not going to cite counterexamples, nor would I suggest that selling a bazillion copies certifies a “masterpiece.” (The international “masterpiece” standard is two bazillion.)
I would, however, point out that the reasoning by which you dismiss country, bluegrass, and hip-hop as American-invented can also be applied to all of your entries. The banjo is generally considered to be of West African origin (cite with a bunch of external links), and comics are dated, depending one’s definition, at least as far back as 1682 (cite and cite). Jazz was as heavily influenced by African musical traditions as bluegrass was by Celtic. Tap-dancing owes much to Spanish flamenco, as well as African and Irish dances. And musical comedy’s purely American origins would be a surprise to devotees of kyogen.
For an art form to be entirely “new” is impossible unless it’s in an entirely new medium. There’s constant interplay and influence among genres. I know of no artists (in any medium) who thought of themselves in terms of a genre; “genre” is a false construct imposed after the fact by critics, in order to compartmentalize an overwhelming volume of interwoven material into comprehensible units. (And, of course, to simplify the creation of university syllabi.)
My point being that if you’re going to assert that rock, country, bluegrass, and hip-hop aren’t American in origin because of their foreign influences, you can’t really claim that comics, jazz, banjo, tap, and musical comedy are either.
The contraceptive pill.
The car.
Carrot Top.
The Impressionist exhibition of 1874.
I thought that contained the word “exhibitionist” at first glance 
Some explanations are in order. For instance, why is the Impressionist exhibition of 1874 the most important cultural event of the last 200 years?
In a general sense, it was the first major cultural event to make the statement “Screw the past. Who cares about what the old farts did? We’re here now and we’re the future of art.” This attitude went on to become a major theme in all forms of art.
Jazz is nowhere near king in America. Jazz is at best our cultural ambassador.
And I play the stuff, so I ain’t hatin’ here.
I trump your pill with my carbolic.
Ahhh, 1979. Gives one a Trevor.
Other than that how did you like the play Mrs. Lincoln?
Leonard Bernstein once called Elvis the greatest cultural force of the 20th century. I could provide a cite for that if needed, but for now it would involve me getting up from my chair. 
If I have to vote for an actual event, being an American I’ll go with the St Louis Exposition. Featuring the invention of the ice cream cone plus the mass-popularization of then-regional American foods such as hot dogs, hamburgers, iced-tea, peanut butter, and cotton candy, the expositions impact on future American life was immense. For entertainment, you had John Phillips Sousa’s band placed next to Scott Joplin’s piano. Recorded music and motion pictures were being showcased and used in the exhibits, and much of the exposition itself was filmed (another first for an event of this magnitude).
In terms of "what is the most important thing to happen to pop culture in the past 200 years that is not an actual pop culture ‘event’ ", I vote for the application of electricity to art. All forms that could take advantage of the new technology (not to mention new art forms like motion pictures) succeeded wildly in the past 200 years, those that couldn’t (say, sculpture) have faded. They haven’t died, of course, but art forms that do not use electricity (or can’t be recorded) are ignored by the masses.
I would say that Enrico Caruso would fit the bill as being the first truly “big” recording star.
However, there were sports figures who were famous nationwide when Caruso broke out, so if you want to count athletes, then perhaps… King Kelly? Or the aforementioned Jenny Lind?
It’s amazing that jazz is incredibly popular in every country but the one that created it.