post). The link however was dead, and so I needed to find another alternative.
Thanks to Michel Merx, I could find the answer in the comments of the site!
So for future reference, to allow HD trailers in Front Row on SL 10.6.6;
Open Terminal.app and enter (copy paste should work)
defaults write com.apple.frontrow EnableHDMovieTrailers TRUE && killall FrontRow
The && killall FrontRow will stop Front Row if it is already running.
Thank you Michel and the internett