If you happen to have extra big display connected to your Mac and you'd like to watch all movie trailers inside your Front Row in HD (High Definition) format, you should try this command line:
defaults write com.apple.frontrow TrailerBypass http://www.apple.com/trailers/home/xml/current_720p.xml && killall "Front Row"
But you need to make sure that you've extra fast broadband internet access, or you would end up waiting for several minutes to stream-play the HD movie trailer. And you need to pay attention that with low resolution display, you won't find any difference in the trailers' quality, so it's better to keep the default settings.
How to download movie trailers faster?