No, it is not. Pixar is an independent film studio, founded by a grup of people including Apple founder Steve Jobs. They merely have a contract with Buena Vista, the distribution arm of Disney, Inc., to distribute their films. That contract has already been set to be terminated after the release of "Cars."
However, I did specify "Disney STUDIOS" to prevent confusion between studios and distributors, since Pixar studios does use Disnsey's distributor.
I had heard they were planning to stick around after The Incredibles but that's just hearsay. Actually Pixar started life as the special effects division of George Lucas's ILM. He sold it to Steve Jobs as part of his divorce settlement in the early 1980s.