1. Keller's company, PA, was bought by Apple mainly for its engineering talent. Getting Keller was a huge deal for Apple.
2. Keller has been hired away by Mark Papermaster, who was a disaster at Apple (all of Bob Mansfield's reservations about him from the internal email discussed in IBM v. Papermaster came true: cultural, pace of development, etc.).
I would anticipate an outside hire from Apple to fill his shoes in the next 6-9 months.