Apple's biggest competitor in the mobile-ad space is turning out to be Amazon, with its fast-growing Kindle device and a burgeoning mobile-ad offering.

The Seattle-based e-commerce giant is already selling ads on its apps and websites, but recent moves point to much bigger mobile plans. It has hired Jamie Wells from Microsoft, where he led global product marketing for mobile display and apps. And he, in turn, is building his team -- his LinkedIn page proclaims: "Hiring for Mult[iple] Ad Sales Marketing Positions." ...