I was browsing /Film as I so often do and happened upon another Pixar related article. This article talks about a French short film, Above then Beyond, that could have inspired Disney/Pixar's film Up. After reading the article and comments, this is what my conclusion.
There is not a shadow of a doubt in my mind that Pixar was inspired by the film and seemingly borrowed elements - maybe even direct screen-by-screen shots - for their film, Up. I do not, however, think this constitutes as blantant "stealing".
After reading Sciretta's - the article writer - timeline of the two films (as far as when things were actually published so to speak), it seems Above and Beyond was out long enough for Pixar to see it and get inspired from it. The students, or company, who did the film even have a partnership with Pixar.
While I do not think any idea is an original idea and most certainly not the idea of tying balloons to an object to fly away, i'm a little let down by Pixar from borrowing so many screen-by-screens from the French short film. That sequence in Up was my favorite part and, although the French film's cimentography and such do not tickle my fancy, they still seemed to have done it first. It does not make me like Up any less, but I'm just disapointed that my favorite part was heavily inspired by something else. Considering I thought the movie dragged at so many points after that, I am let down even further.
It seems that both Disney and Pixar draw heavily from other things. That does not make either any less of an amazing coporation for entertainment and film. As a fan, I just wish they were a bit careful about how they put their inspiration into their films. I am sure not many people knew about Above then Beyond - I know I did not prior to seeing it on /Film - but I really do wish Pixar would have mentioned being inspired by it. It does make them look incredibly like theives for not saying they were, especially after some of the comments Pixar made that seem contradictory to each other.
