DTS-ES and Dolby-EX encoded DVDs would best take advantage of a 7.1 system, and my favourite demos are LOTR:Fellowship of the Rings Extended Edition (the opening battle scene, mines of Moria) and Star Wars episodes I (pod race) & II (opening scene and dogfight in the asteroid field). For music, I like to use several DVD-A: 1812 Overture (Telarc recording), Flaming Lips, and Bela Fleck. I'm still waiting a few AIX titles to come in from Amazon, but their recordings are highly recommended by many.
I personaly like to listen to the opening scene in gladiator it really emphasises the 7.1 and you can hear every chain link when it moves on the catapults
LOTR II - Extended Edition works well for showing off your stuff. Just the opening scene with the Balrog is pretty intense. Yes, the Pod Race is always cool because you hear the pods go completely around you in a circle.
I've ordered the Peppino D'Agostino and Laurence Juber discs. I'm dying to hear how guitar music sounds on DVD-A, especially on highly regarded AIX recordings. Unfortunately, I'll have to wait til the end of February for Amazon to get it up here (Canada) and then over to me.