It would be feasible for Apple to do this but I don't believe it will happen. The cost to Apple of maintaining a range of processors for its PCs would be large, and unlike with Intel, that cost would not be spread over the whole PC market.
However, having the option to do this keeps Intel's pricing honest (as do AMD's recently improved products). So it may be worth it for Apple to throw a bit of cash at a few well publicised projects in this area.