DualEdge: > Actually, it was Pixar's first 3D movie.
We saw it theatrically in 3D. Christie DLP. RealD passive.
My guess at the time was that Pixar probably started production in 2D, and then was commanded by The Mouse to switch to 3D. That's pretty straightforward for fully modeled CGI content. They might have had to scrap some digital mattes (which would otherwise be too obviously flat).
The use of 3D in "Up" was not compelling except in a couple of shots, and whatever 3D added did not compensate for the luminance loss. "Up" in 3D, as with all the 3D I've seen so far, was visually deeper but darker (in both senses of the word) compared to the 2D.
The darkness is entirely due to the limitations of the majority of today's stereovision projection and display technologies. Until that gets fixed, I'm in no hurry to see any more 3D.