In order to watch trailers in HD in Front Row, start Terminal.app and type (copy paste should work too) the following;
defaults write com.apple.frontrow TrailerBypass http://www.apple.com/trailers/home/xml/current_720p.xml && killall "Front Row"
Basically, what this does is to write a value (http://.../current_720p.xml) to a property (called TrailerBypass) used by Front Row (com.apple.frontrow) and then (&&) restart Front Row (killall "Front Row")
To undo the above type this in Terminal.app
defaults delete com.apple.frontrow TrailerBypass && killall "Front Row"
In some cases this is not reflected in Front Row, if so, start Front Row (cmd+esc) and exit again. Then try to enter the command again.
If you have trouble pasting it into Terminal.app, paste it to TextEdit.app and make sure it is on one line only.
Or as a last resort install "Front Row Trailers" which enables this same feature with just a few mouse clicks. (Don't take me wrong, this is a great app, I just don't like to install more than highly necessary).
Update: I can't make this work on my iMac 24", but still it works on my MacBook.. If someone out there knows a hint or two, drop a line in the comments.
Update 2: Ok, so I finally took a bite of the sour apple (oh, well, you know what I mean) and I installed the preference pane "Front Row Trailers" and now it works on my iMac as well.. Sweet stuff.
Update 3: Now the link in Update 2 is down, see my new post for more tips.