The theory I'm getting from various people is that they're doing this because IBM aren't prepared to commit to long term support and development for Apple's specific needs any more. Apple's computers are a very small part of their chip business, and IBM can simply live without it, whereas Intel want the business and are prepared to let Apple take a hand in specifying the next generation of processors. Also Intel are building in a lot of support for multimedia applications that Apple need, and IBM just don't want to know, since they would much prefer to develop their server business. So for once Intel aren't the villains, neither are Apple.