Although third party AirPlay support debuted as part of the recent 4.3, apps that offer that support are now trickling in. AirPlay allows you to beam video from your iOS device to an Apple TV or to a 3rd party application like Banana TV without cables. The newly updated IMDB application (version 2.1) offers Watchlist, a way to keep track of movies that you want to watch, enhanced showtimes and ticket purchasing, and AirPlay support for trailers.
Unfortunately, you cannot use the new AirPlay features to watch Hulu or Amazon Instant Video, the way you can on desktop browsers.
Update: Looks like the Fandango app also supports AirPlay for trailers. Neato!