The next version of osX (Leopard) will provide the ability for designers to use SVG files and ‘Resolution Independant’ Graphics in their websites.
SVG files are scalable vector files that right now can be viewed on apples Safari browser (like a pdf in a web browser)

Resolution Independant graphics, a new Quartz-driven technology will soon let users choose between viewing more detail (more pixels per point, but fewer points on the screen) or a larger user interface (fewer pixels per point but more points on the screen) without altering the resolution of the computer’s display. Im thinking of it like the Mac osX dock zooming thing so that it doesnt hurt my head.

In effect this means no more worrying about covering different res screens, from 800×600 up.
The quality should ’scale’ perfectly to any screen size.

(NB. I realise Flash does this now, but its graphics pixelate, these will show a higher quality version as the res gets higher.)
(Also I dont pretend to be a systems architect so my discription above may be skewed, in which case here are the real techies talking about it:

High DPI Sites Blog
Leopard feature overview
Apple Resolution Independant UI (the basis of this story system wide)
The system element for techies on apple developer site