3840x2160 is a lot of screen real estate and it's a refreshing step in the right direction compared to the useless low-res crap that we're being offered today. But a 16:9 form factor is still wrong for computers - at least when they're being used for real work. 16:9 is fine for people who think computers are for watching movies and Youtube videos. Give them their glossy "hi def" 1366x768 displays, but the rest of us would be better served by 16:10 displays like the rumored MacBook Pro 2880x1800 display. Yeah - I know 2160 pixels of vertical resolution is more than 1800, and I'd probably choose it because it does put more information in front of me, but some of the utility is lost in the shortscreen form factor.
I think you're misunderstanding how retina display works: Apple uses pixel doubling for that, and indeed that's the only way a 300 dpi resolution makes sense. The effect is that you don't see pixels - everything looks perfectly sharp.
But you don't see more information - the letters are the same physical size as before, so are icons, and so on.
If they were just putting a 300dpi display on a current laptop - without doing any pixel doubling - you'd end up with tiny, unreadable text, and tiny icons.
This is yet another example of Apple improving the consumer experience using high tech. Other companies maybe wouldn't have thought of it - after all, you don't show *more* on the same screen, you just show the same information *better*. And it requires a huge effort on the backend, of course, with the whole OS having to be ready for this, and graphics power expended just to make things look better.
Before I am called a blind devotee, keep in mind that the iPhone 4 / 4S is the only pixel doubling device on the market. Android doesn't have the capability, although they get pretty close by scaling the fonts up and blowing up the icons, and making displays with more pixels - but they're not pixel doubling. If you go to laptops, this isn't really possible on Windows.
These things will come but I am willing to bet that the MacBook Pro retina display will come only *after* Apple's released an iPad with one. There are a couple of reasons: iOS can already do pixel doubling. The iPad is way more important to Apple than laptops (it's their play to take over the world, by owning the post-PC market and betting on it displacing the PC market). And the iPad has a smaller screen so manufacturing is easier: The smaller the screen the higher the yield.
And yeah I want one now.
