Proper DPI scaling has been in Windows for a while (since Vista or so), but the main problem was a lack of screens that would push the resolution up that high... at an affordable price range for the mass market. As a result, developers didn't bother, and save for the very specialized softwares, most will just scale texts well while having their images squished or scaled.
It's only recently that such display technology has come down in price enough to make an impression. Still, I anticipate that it won't be a mainstream thing until 5 years later, where every device made has a high resolution screen.
But from a developer's standpoint (I'm a multi-platform developer myself), I think... developers should have worked with Retina displays long ago. Or as soon as possible. Once the technology hits the mass market, those who can come out first with software solutions that take advantage of the technology will profit the most in the long term. Just look at the iPad. Apple was first to market with the technology, and just 2 years later, they've racked in billions.
If profits aren't your main goal, you'd still secure a place in the market that's really hard to displace. It's truly a classic example of "the early bird catches the worm".
