It is likely that this would happen sooner or later, with Star Wars and Pixar both in the hands of Disney, but it looks like the signal has been given for Pixar to get to work on their take on an animated Star Wars movie.
Pixar's release schedule is packed at the moment, with Inside Out, The Good Dinosaur, and Finding Dory already in the works, so it's unlikely that we'll see this any time before 2017, but if rumours are true then this would be Pixar's first movie based on an existing franchise. Hopefully Pixar still get to have their take on it, and we don't just end up with a standard animated film that could have been done by anybody.
To be a true to Pixar's spirit in an existing property may prove difficult, but I'm sure there are a lot of fans who can't wait to see what this combination would turn out like. With Disney planning to release a Star Wars movie every year, it's inevitable that they will have to use all of their assets to keep the ball rolling.
Of course, rumours are rumours and until official word is released by Disney or Pixar, it is anybody's guess what will happen in the future.