Doesn't have to be mirrored.
I have Front Row working on my larger external monitor with my iMac. You just have to set it up so the external is the primary monitory, i.e. where the menu bar and dock live. Go to the Display Pref Pane, arrangement tab and drag the menu bar between monitors to change which one is primary. I have no idea if this has and bearing on the original poster's problem, but it wouldn't hurt to fiddle with the setting.