The bottom line is that at the distances people view their desktop monitors, they don't want the buttons, graphics, and text to be any smaller than they appear with about 80-100 ppi screens. Give all but the most recent applications and operating systems a 130 PPI screen, and there will be UI items that don't scale. Some UI items, like text, will, leaving the interface feeling out of proportion. Higher PPI screens are available on laptops because 1.) Some people demand the implied screen real estate and 2.) Laptop screens are generally closer to your face. I won't discount the whole 1080p standard putting a natural cap on many screens these days (though my 24" BenQ is one of a dwindling set of 1920x1200 panels).
This is why when Apple put high PPI screens in the iphone and ipad, it doubled the PPI. Existing apps would look the same*. Apps can trivially use perhaps the single greatest feature of high PPI: more crisp text with less dependence on antialiasing to mimic round corners.
And it's why, I suspect, if Apple does release Macbooks this year with "retina" displays, they will be double the PPI. While Mac OS X in theory supports a reasonably scalable UI, applications may not. And web browsers will want to operate as if they're rendering in the lower PPI, though rendering text and non-bitmapped elements at the higher resolution. Eventually (maybe next year), we'll see expensive Apple Cinema displays doing the same thing. And there will be the normal competitors (especially Dell).
But until recently, a 150 or 200 DPI LCD was crazy expensive to produce. Judging from the ipad 3, it also takes significantly more backlight capacity (provided by very bright LEDs in that case). We're just now entering a stage in which there are rumors about the 11" and 13" Macbook line getting them, maybe the 15". It will be a while before the 27" and 30" panels can be produced at a price people are willing to pay. That said, I'm holding off buying any more monitors or replacing my T series Thinkpad until they're available. I'm hoping I don't have to wait past 2013.
* Or at least ought to. Some apps on a third gen ipad will actually look fuzzier than it should.