Henrik M. posted a neat solution on Apple’s discussion board to to have Front Row display video on an external monitor or TV. First, you have to get the ID of the external display using this utility: http://rapidshare.com/files/54473723/displaysInfo.zip, and then enter the following command in a shell:
defaults write com.apple.frontrow FrontRowUsePreferredDisplayID 12345678
where 12345678 should be replaced by the secondary display ID. It did try it on my MBP using Leopard (OS X v10.5.6), and it worked fine.
From what I understand, it is possible to revert this setting with the following command:
defaults delete com.apple.frontrow FrontRowUsePreferredDisplayID