It's been speculated about for at least two years now. I've been saying that Apple has something in mind by staying with two cores in their SoC. The A8 isn't powerful enough for OS X by itself. But, if Apple used two of them, they could do it. If, as it looks like, the new iPad will have an A8x chip, that would certainly help. Two cores, because most computers don't need more than four.
Let's look at what the problem is, and a possible solution. One chip will equal the top Bay Trail, but not an i3 low power. Worse, the lowest chip Apple uses is the i5 low power. That presents a problem. But so does the emulation needed for third party software. How many developers will be willing to rewrite their software for yet another OS X chip change? Microsoft, Adobe? I doubt it, particularly as only the lowest models could use these chips.
But if Apple uses two, the power usage would still be lower, the compute would be adequate. Apple could add the few instructions to the chips that cause most of the emulation slowdown, allowing Apple to run OS X and their own apps natively, and whichever others that would need to under emulation. The cost would be much less, because at an estimated $27 per SoC, two of them would be less than half of the cost of the cheapest Intel solution Apple is using.
This is complex, and I'm not sure this year's chip is quite enough. It will be interesting to see.
As far as the retina screen goes, they're likely too expensive, and need a lot more GPU power than a Macbook Air has, as there is no separate GPU. That will be an interesting challenge as well.