Dougofthenorth,
From what I've recently read, officially recievers not THX certified could not decode Dolby Digital Surround EX having apparently something to do with lucasfilm thx developing this. However, this option will now
be available to non THX certified recievers under the name Dolby Digital EX. Source: Sound & Vision Magazine, January 2002, middle of page 16 in a/v digest.
Also a good brief overview of home-THX/specs can found at the following website for those intrested and might save you some hunting around:

[Link: multimania.com]