Iron Man 2 viewers will get a surprise treat this weekend — an unheralded 90-second trailer of Super 8, a mystery film project that J.J. Abrams (Star Trek, Cloverfield) and Steven Spielberg are producing. Confirmed details on the movie (scheduled to start shooting this fall) are few, but internet bootleg copies of the trailer have yielded some clues. Here's what online sources are saying so far (spoiler warning):

What does the trailer show?
A car swerves onto train tracks, derailing a train which crashes in a burst of flames. As the camera wanders through the wreckage, it zooms in on metal doors with a U.S. Air Force logo; something trapped inside is trying to punch its way out. "Next Summer," reads a superimposed title, "It Arrives." 

What is "it"?
You'll have to wait and see. According to The Hollywood Reporter, Super 8 is a supernatural drama about a group of late-1970s kids who "accidentally capture something... sinister, on film" while fooling around with a Super 8 camera. reports rumors that the film will pay homage to Spielberg's Close Encounters of the Third Kind and E.T.

How did Paramount keep the trailer a secret (more or less)?
Locked canisters containing a reel of the exclusive trailer were
distributed with prints of Iron Man 2. Projectionists were not given the combination code to unlock the canister until late Thursday.

Has this kind of "surprise" marketing been done before?

Yes. Abrams piggy-backed a vexingly vague teaser for his monster movie Cloverfield on 2007 screenings of Transformers.

Does it work?
Despite mixed reviews, Cloverfield made $168 million. Paramount is clearly hoping Super 8 will do the same when it goes up against likely blockbusters Transformers 3 and Captain America in summer 2011. 

Could it be a sequel to Cloverfield?
No. Despite rumors to the contrary, both Variety and The Hollywood Reporter say it is a standalone project. As for Abrams, he's consistently mute about his projects, especially his TV series "Lost," renowned for its inscrutable, twisty plotlines. "He'd rather maintain the mystery and let the images speak for themselves," say Ace Fernandez and Borys Kit at THR

Sources: The Hollywood Reporter, Variety, SlashFilm