To enable high-definition trailers in Front Row, type the following command in Terminal:
defaults write com.apple.frontrow TrailerBypass \
http://www.apple.com/trailers/home/xml/current_720p.xml \
&& killall "Front Row"
To reset to default settings, type the following command:
defaults delete com.apple.frontrow TrailerBypass && killall "Front Row"
Obviously, this requires a broadband Internet access if you don't want to wait for several minutes before the high-definition trailers start playing. If you want even more control over the trailers in Front Row (French, German, Italian, Belgian, Québecois, Danish and Japanese trailers; sorting by release date or by title), you may want to try Front Row Trailers, which is basically a GUI to this hint.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007101906541184