My Denon registers Dolby Digital EX for nearly everything that I play these days. There's a reason for this which I posted in another thread but can't find right now. Anyway, to quote the Dolby site again:
Feature films originally released in Dolby Digital Surround EX (the cinema version) carry the encoded extra surround channel in their subsequent DVD releases, as well as onto 5.1-channel digital satellite and TV broadcasts. If your home theater system has a receiver or preamp/processor with Dolby Digital EX decoding and speakers to support 6.1 or 7.1 playback, you can hear Surround EX soundtracks as they were meant to be heard, with the increased realism created by the extra surround channel. As in the cinema, with regular 5.1-channel Dolby Digital playback no sonic information is lost (although you'll miss out on the heightened realism).
Current Dolby Digital Surround EX soundtracks contain a digital flag that can automatically activate the EX decoding in a receiver or preamp/processor. For titles released prior to late 2001, however, you need to turn on the EX decoding manually.
Basically, EX decoding doesn't work on 5.1 receivers because the flag isn't recognized. And even if you have a 6.1/7.1 receiver but only a 5.1 speaker setup, the EX information becomes irrelevant. You really need a rear surround channel to properly deliver the matriced soundtrack.