This is pure rumor for now, but one that is not entirely baseless. Pixar’s owner, Disney, also owns LucasFilm now and The Clone Wars proved that Star Wars will work in CG form, so it’s not entirely unsurprising if people do consider it a possible that Pixar is hard at work on a new Star Wars film.
The source of the rumor is Latino Review, which has a pretty good reputation for being right about their speculations and rumors. LR’s report has not provided any details yet other than stating that the animation studio is working on a Star Wars movie right now. It should be noted that the Star Wars movie, if the rumors were true, is separate from Episode VII, which will be live-action. (Actually, the rumors didn’t say that the movie is going to be CG. Everybody just assumed that because hey, it’s Pixar.)