I think that the best thing to take to you for any demo is anything that you know well, and moreover know what it is suppposed to look like, and therefore you have some reference point. Monsters Inc. is of course great eye candy, but it actually is on just about any device IMHO.
Having said that, the things to look out for (for me anyway) are movement (both of images on screen and also panning shots), white/light background (for any panel structure visibility), contrast in dark scenes, and then overall depth of field/sheer filmicness (oops that's not a proper word

). For me, and as much as because they're discs that I've played many times before and have things I know what to look for, I'd say a combination of Saving Private Ryan, Band of Brothers (snow scenes in 'Bastogne' very good for showing a pj's ability to display white as it should be), Fast & the Furious for movement, Gangs of New York for panning, LOTR:ROTK (Frodo in Shelob's Lair) and Dances With Wolves (when they find the buffalo) for filmic images (another good scene to show this well which will really show the difference between good and bad projectors is 'Points' in Band of Brothers, where they are across the Lake in Austria.)
Just my 2p