In the past, Apple has been sensitive about selling computers with very high DPI screens on which the menus and buttons are so tiny that they are hard to use. And so they've often avoided the highest DPI screens and most Apples are in the 100-120 range. In contrast, Windows manufacturers have tended to offer more models and given users that sort of configuration if they wanted it.
Resolution Independent UI is a next generation sort of feature that is widely rumored to be in Leopard. What this feature does is use a combination of scaling bitmaps (like OS X does with icons in the dock now) that can be scaled to a limited extent, and vector-based buttons and interface widgets that can be scaled arbitrarily. The entire user interface can then be rendered onto the screen based on desired physical size -- rather than showing up on the screen pixel-for-pixel and being whatever size that dictates.
Once you have this feature, you can make the screen DPI as high as you want without impairing usability.
To get a picture of where this leads in the long term... think about getting a 300DPI screen on a laptop, if this were possible. It would have 4-9x as many pixels as a screen today. If it were put on a laptop running Tiger or XP, then it would be unusable because the menu bar would be teeny tiny, etc.
But with a resolution independent UI, the menu bar and widgets could be just as big as they ever were (but look extra gorgegous with more pixels to render them). At the same time, the picture or document you're working on will have 4x - 9x as much detail as it did before....
Anyway, my theory is that you'll see Apple migrate to top-of-the-line resolutions after they get this feature, because they're too serious about ergonomics to go for unusable DPIs...
