Keller could be leaving for a whole slew of reasons. At AMD, he works for a company whose core competency is chip design - his strength. At Apple, he is just a cog in a giant wheel. If he is instrumental in an AMD turnaround, he would have made a bigger impact than if he just pushing status quo at Apple.

I'm not sure this is a major impact for Apple. Firstly, I haven't seen anything groundbreaking come from Apple on the chip side. Secondly, there are plenty of talented people from AMD, Intel,HP, etc who would love to work for Apple. He can be replaced.