From the Google blog
<quote>
Just search for the movie name and your Zip/city on Google.com, and the first result will include a link to the trailer, as well as critics' reviews and showtimes for your local movie theater.
</quote>

Link:
http://googleblog.blogspot.com/2006/08/now-playing-movie-trailers.html

It's done by using the 20% time.

yes don't forget the [officially], because one year ago this feature was already there...
http://blogoscoped.com/archive/2005-02-24-n88.html
http://www.google.com/intl/en/help/features.html#movies

I almost didn't post that part, but I knew I had seen it before so I added the offical part :)