I think that Retina resolution displays will dominate the field in much less time than ten years: Since every mobile device maker has to match Apple I'd bet that by next year every new phone and tablet will feature a higher resolution screen. maybe that won't be 256, but maybe it will be at least 150. Then you can bet that in at least two years you'll see the same thing happen on laptops — and desktops will have to follow within five years.
The truth be told: It's sort of shocking to think that we've been using 72/96 for so long, I would have expected this shift to have happened ten years ago.
We're only just now reaching the point where I can count on my friends having 64-bit CPUs.
Intel's first 64-bit mobile processor was released in mid-2006. That is only six years ago. The manufacturing transition itself was pretty quick - only a year or two until all their currently-manufactured CPUs were 64-bit [AFAIK], if we ignore Atom. Many of my friends have 6-7 year old laptops because they still work and that's all they need.
64-bit was chiefly engineering work: the CPUs didn't cost more to make because of Moore's law [I believe]. I hope that Retina screens are similarly not-more-expensive when made in large quantities. But what if they are a bit more expensive? We'll be cursed with budget 72/96 screens for maybe five more years on top of the length of time people keep their devices!
And it really will be a curse to us, personally, web designers. The Web uses images, and everyone uses the Web.
Despite the fact that many people primarily associate "Retina" with the iPhone (because that is where it was introduced first), I think this is the area in which it has actually had the least impact. Other phones had high DPIs before the iPhone 4, and the difference between the best Android displays and the iPhone 4 display was not overly dramatic.
OTOH, pushing laptops and tablets out of low-DPI land is a truly remarkable achievement. Other platforms are much farther behind in this area, IMO, and at least on laptops have a much longer path to catching up.
So basically 15 ppi, then - your comment reads as someone trying to exaggerate the difference - the T220 was 205 ppi, and some of Apple's Retina devices are 221 ppi, a grand difference of merely 7% - well, seven per cent and eleven years.
The truth be told: It's sort of shocking to think that we've been using 72/96 for so long, I would have expected this shift to have happened ten years ago.