Summit can't honestly believe that every person who walks into New Moon has read all four books in the series. I have not read any of the Twilight books. I have seen the first film and will likely see the second one out of curiosity (the series makes for excellent writing material). Unless this teaser contains no footage and no real hints of narrative, it will have the inevitable effect of spoiling the finale of the second film before said movie even starts. Just for an example, it would be like sitting down to watch Harry Potter and the Goblet of Fire and having to first watch a teaser for Harry Potter and the Order of the Phoenix, one which explicitly details the return of Voldermort and/or the death of Cedric Diggory. Or imagine sitting down to watch The Empire Strikes Back and first watching a teaser for Return of the Jedi that had Luke, Lea, and some guy named Lando gearing up to rescue Han Solo from the evil Jabba the Hut. If Summit wants to run a teaser for Eclipse right after New Moon, that would be both appropriate and welcome. But for goodness sake, I sincerely hope that they are no going to spoil the second movie for every paying customer before it even starts.
UPDATE - A box office contact who works with Summit has stated that there is no intent to run an Eclipse teaser before New Moon. Glad to hear it.