I don’t know that Pixar needs be worried yet, but those Shrek and Ice Age cats ought to be worried about Big Buck Bunny, the story of a large, gentle-souled rabbit pushed just a wee bit too far. It’s done entirely on Open Source tools (Blender in particular) and, to be quite frank, is an amazing example of what can be done by a dedicated few. It’s also Creative Commons licensed, for those that care.
Check it out:
Very cool. Also note that the HD version of the short is available, which ought to be viewed for the full effect.