Sony and many other LCD projector makers have been making high pixel density LCDs since the 90s (think 1024x768 on an square inch). And there has been a smartphone out shortly before the iPhone 4's release date that had a 300+ dpi by some major manufacturer. LCD's major cost isn't pixel density, it's the size of the glass. The reason why they weren't released is because nobody was willing to buck the PC status quo of not having resolution independence (or doubling, like apple did it) since it would miniaturize everything to be irrelevant, making it not sell well. People who needed to see a lot of detail got specialized displays.
Plasma displays it's more the pixels, which is why large 720p plasmas are cheaper.
Samsung is also a huge company with south korean government backing, they can raise the capital to make whatever improvements apple contracts them out to do and make a shadow factory at the same time.
Ultra high res displays are extremely expensive - 4K monitors are over $10k. Apple will democratize them by having a better manufacturing process to make them a few hundred or less. Plus, i would bet Apple can bake in resolution independence into OSX far quicker than Redmond will.
Typical chicken/egg problem. They're over 10k because of low production runs and they're targeted towards industries that spend big, like the medical industry. If they were sold to the mass market then they would be close in price to your typical 32" LCD TV. Why are 16:9 ~21" 1080p screens so cheap, because they're parts from small LCD TVs! It's really the lack of demand that causes it. The sony LCD projector screen I quoted for example (it didn't have a backlight, it was just the lcd), you could purchase for around or under a $100.
Plasma displays it's more the pixels, which is why large 720p plasmas are cheaper.
Samsung is also a huge company with south korean government backing, they can raise the capital to make whatever improvements apple contracts them out to do and make a shadow factory at the same time.