Apple recently filed the patent US 20050285965 entitled User-Interface design. This patent describes a way to render UI elements on display using a different resolution than the one used at design time. This would allow for instance to define a widget with a set of properties (shadow, whatever) on your 96dpi screen and render it flawlessly on a 120dpi screen. The engine behind all of that seems to be, from the patent diagrams, a simple rasterizer. Yet, the patent also provides (crappy) screen captures of the designing tool with examples of properties. (outer shadow, outline, highlight, etc.)
Given the rise of high-dpi screens it's important we start having toolkits to support them. Microsoft seems to have a pretty good solution with Avalon, although I never actually tried it on different resolutions, and MacOS X already provides some kind of resolution independence support. If you have installed Xcode, run Quartz Debug and play with the zoom feature.
Thanks to MacBrains for the link.