If you made the new iPad 2012's screen—with a 2048 x 1536 pixel resolution—with the pixel size the original Macintosh, this is how they would look together. Pretty impressive, right?
Click images to expand and see at 1:1 pixel resolution.
The original Macintosh, released in 1984, had a monochrome 512 × 342 pixel display. That was 175,104 points. The pixel density was very low: 72 dots per inch. The iPad now has 3.14 million pixels. At 9.7 inches, that's 264 dots per inch. 3.66 times as much pixel density. If you made the iPad at 72dpi, then you would get a tablet the size of a large TV (Gene Munster just wet his pants now).
Here are some more staggering numbers: 28 years ago, a monochrome icon on the Mac was 16 x 16 pixels. It only took 32 bytes of memory. Compare to the 512 x 512 pixels of each iOS' icon (required to go on the App Store). It takes four times the total video memory of the original Mac to represent a single icon in iOS at full size. Of course, iOS' icons are not shown on screen at that size. They are much smaller—114 x 114 on the iPad. Eventually, however, you can be sure that there will be displays that would require that insane pixel density.
What is remarkable is that the absolute size of the icons remains more or less the same. It's an amazing increase in the density of information required to represent the exact same object with the exact same meaning. Eye candy is expensive.
If anyone had told me this when I tried my first Mac, back in the 80s, I would have never believed it.