Resolution Independence: It's about the details
Wow! there's a whole lot of ignorance about RI in the previous comments.
Resolution Independence (RI) is all about SCALE of the User Interface (UI). Both Raster and Vector graphics are used in Apple's RI implementation for excelent reasons.
RI has one user accessible control, and that's the Scale slider. It snaps into place at 1, 1.5, 2, and so on. This feature is not Final, nor is it officially announced.
In an RI environment, we talk about Units, not Pixels. When SCALE = 1, then 1 Unit = 1 Pixel. Apps that don't use the new RI aware programming methods are unaware of the true pixel resolution of the display, they only know the "unit resolution" of the display. So they're output is blocky compared to RI Apps, but no worse than they already are. In other words, you don't know what you're missing yet.
You can already change the SCALE of your display in a NON RI way by lowering the Resolution of your "Video Card" output. Then your text will be bigger, but not clearer. Keep in mind that "Bigger" is a perception, based mostly on distance. If you can't see all the detail on screen (if you're sitting far from the screen) then you may be able to get closer to the screen and see those details, or use OS X's Zoom feature (System Preferences / Keyboard & Mouse / Mouse / Zoom) to enlarge part of the display. But once you see all the detail, getting closer, or enlarging it more, won't make it clearer, just bigger.
Here's how RI works. Lets say we double the resolution on a screen. Now we have twice as much space to place our windows and icons, but they are half the size they were. If our eyes are good enough, or we're close enough, we can see all this detail, and life is good. But if we need things to be larger in an RI enabled OS, we can increase the scale. To get back to the original "size" on the screen, we set the SCALE to 2. Now everything looks like it did on the original screen, with one difference. The system can display extra detail to sharpen what we're seeing. We may not be able to see all of that extra detail, but we can enjoy all that we are able to see, and now we can lean forward and see more. For many people, this will mean they can lean back and actually see comfortably. And I mean us normal people, not just folks who are legally blind.
Some images can be redesigned with Vector tools and they'll be smaller, and scale perfectly. However, some images need to be photo realistic, and they'll still be raster based. Many icons are going to be raster based and developers will need to provide multiple resolution versions of their icons. So RI will actually cause some Apps to end up a little larger, while others will be smaller. But icon size is negligible for an average size App. If developers include an icon for every scale setting (which is optional) they're not likely to reach 500 KB. That's less than .5 MB.
The issues in OS X's RI are said to be alignment issues. So some elements on screen may overlap in some current Apps, currently in the "developer only" test environments, and user's are NOT able to turn on RI normally (requires hacking) so RI support in Leopard is provided for developers to prepare their Apps for the an RI enabled OS, possible to be released in the near future.