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.

[robg adds: I tried this on my MacBook Pro, but to be honest, I'm not sure I noticed any difference. If someone can try this, and find a trailer where there's a visible difference, post in the comments. I tried a few trailers from Apple's HD Only Movie Trailers page, but they all seemed to look the same to me, regardless of the above setting.]