Your Friend the Rat
I don't own a hi-def tv, or a blu-ray/HD DVD player, but Hi-def 3D animation is definitely a good case for the extra resolution.
Case in point, apparently the Blu-ray edition of Ratatouille will feature an exclusive short, "Your Friend the Rat", directed by Jim Capobianco (story by Jeff Pidgeon, and Alexander Woo).
[Update] The user reviews on Amazon seem to point to the short being on both the DVD and Bluray versions - we'll see when it comes out, I guess!
Here are some stills I've found:
I'm not sure how many of you read my "News from the Grapevine" section to the left, but in related Pixar news, there was some Wall-E test screenings in Portland recently, with one account posted here. Apparently the film looked 80% finished to the viewer.
Also, according to the New York Times, Sony Pictures Animation and Sony Pictures Imageworks may be up for sale.