Apple's next iteration of OS X, Leopard,
will may be (but probably will not be) the first major operating system that supports "resolution independence," the ability to display the interface based on relative size, not simple pixel counts. By freeing the interface from to-the-pixel mappings, computers will be able to more easily take advantage of greater "pixel density" in monitors.
See, right now LCD panels on computer screens run in the 90 to 120ish pixels-per-inch (PPI) range. Obviously, the bigger the display, the lower the pixels-per-inch, given the same resolution. (That's why those humongous 1080p LCD televisions, nice as they are, look more "pixelated" when you're sitting right up next to them compared to an LCD monitor with an identical 1,920 by 1080 pixel resolution. Of course, you're not supposed to sit right up next to a giant LCD TV.)
Higher PPI displays exist; I keep hearing about a 300 PPI monitor from IBM, although I can't find a link. The iPhone has a relatively low-resolution screen, but a very dense 160 PPI, which is why images look so sharp on the phone.
Of course, if you output one of the current-generation operating systems to one of these high-PPI displays, all your icons, bound to specific pixel heights, will look tiny. (A one-inch icon on a 100 PPI screen would obviously be only 1/3rd of an inch on a 300 PPI screen.) By adding resolution independence, an operating system can scale its output up or down depending on a screen's pixel density. That means something that is meant to be an inch tall is always and inch tall, and text that is supposed to be displayed at a certain size and weight will always be the right size and weight, albeit more crisp.
But if you add resolution independence, you'll need to make sure that all your source images are very high resolution. The easiest way to do that is to save them all as vectors—line shape data instead of "raster" images—or you can just do what Apple did and re-render all your icons to be huge images, like this one for TextEdit. (And I actually sized this one down just a bit to slot it in.)
I really didn't intend to explain anything about resolution independence when I started to link this. Next time you're just getting a picture and a link!
That Answers That [NSLog]