Just to throw an alternative out there, I would recommend any or all of the Pirates of the Caribbean films. They demonstrate superior image and sound quality, and I find them to be very entertaining as well.
If you like infectiously-fun-and-yet-moderately-serious musicals (and want something for more general family entertainment), Hairspray might be a good choice too -- it gets a lot of repeat play in my household w/ 2 grade school kids (and even a toddler who also loves to boogie to it ).
I haven't seen Enchanted yet (though I plan to do a blind buy at some point), but that might be a great choice also judging from all the rave reviews.
But yeah, I'd second most of the other suggestions as well. It really depends on what kind of film you feel like using for that "maiden voyage" w/ your brand spanking new HT.
Personally, I'd rather go w/ Ratatouille or maybe Happy Feet for overall entertainment value instead of Cars myself (though the Happy Feet BD unfortunately only provides a 640Kbps DD track :frowning: ).
I don't know about these latter recommendations; don't get me wrong--said animated Blu-rays do look stunning but, IMO, the very best of standard DVDs were always the DVDs of Pixar films, so the quality gulf between them and their Blu-ray counterparts isn't as wide as it is for most live action films.
PE is the first thing we put on when the new TV arrived and got hooked up on Monday night. The first episode, From Pole to Pole, was so gorgeous I was on the verge of crying. Engaging, beautiful, extraordinary. And educational, too!