My Denon receiver allows me to specify that if an EX flag is not detected, it applies DPLIIx to extract a "rear center" from the existing surrounds. My understanding is that it does about the same thing (more or less) when extracting a true EX center rear, so I'm covered either way: if it is EX...
Doug, you didn't have anything set up wrong. My Denon detected the EX flag for the new THX trailer, but not for the feature. Me thinks somebody messed up (on a Star Wars disc, too.... Imagine that....;))