It's believed that Apple would use AMD's chips to increase its flexibility and broaden its competitive options. Having an additional source for processors could also be handy. Recent rumors suggest that supply issues with Intel's mobile Core i5 and i7 processors are responsible for delaying a MacBook refresh.
Another major factor is AMD's attractive integrated graphics solutions -- especially considering the chipset licensing feud between Intel and Nvidia. Intel has blocked Nvidia from making chipsets for any Intel processor with an integrated memory controller -- essentially all of the company's latest CPUs.