The movie trailer is here!!! Just click on http://www.youtube.com/watch?v=sBGbKCm_pQQ. It is scheduled for release in the U.S. on November 21, 2008. (Thanks, Mollie, for the tip!)
Hey, I just noticed that Rotten Tomatoes already has a trailer analysis up at http://www.rottentomatoes.com/m/harry_potter_and_the_half_blood_prince/news/1745198/rts_trailer_breakdown_harry_potter_and_the_half_blood_prince but surely if you have read the book it's okay to read it.