Ever since Apple finally abandoned the PowerPC platform in favor of the x86 architecture, Macs have been exclusively powered by CPUs from Intel. Throughout that time, however, there has been speculation that Apple could dual source and use some of AMD's chips as well; that may have just been posturing in order to get a better pricing deal from Intel, though. So far, AMD's only inroad at Apple has been through its ATI graphics division, which has supplied Radeon GPUs for an array of Macs over the years.
The AMD speculation has flared up again in recent days following an analyst meeting to discuss the company's new Fusion chips. Fusion is a platform that AMD has been working on ever since it bought ATI several years ago. The Fusion chips combine the CPU and a GPU onto a single die, and one of the slides in the AMD presentation featured a lineup of iMacs and a Mac Pro. The slide apparently went by without discussion, and neither Apple nor AMD has made any announcement about Fusion chips appearing in future Macs. AMD has begun providing Fusion samples to hardware partners for testing, with full production due to start in early 2011. It would be a shock if Apple doesn't at least test AMD chips, but seeing them in production will likely require a very positive combination of performance, power consumption, and pricing. We'll just have to wait and see.
[The source on this was incorrect, and the speculation is merely that -- there is no mention of AMD powering the CPU's of future Macs in this presentation - Ed.]