well its kinda of a new phenomena. My girlfriend wrote the Kept Man and that is really how I found out about them. A lot of them are cheezy photo montages with the author narrating - I wanted to do something different.
Many publishing companies use them as part of their marketing strategy - to broaden their books exposure. I think for certain books it works pretty well. If your interested in doing one your best bet is to find a new up and coming author with a book you like and go to town. I think its most important you like the material or at least have a interesting way to visually translate it.
If you got to the Riverhead Books site you can see some
other examples of trailers:
us.penguingroup.com/static/html/riverhead/index.html