I had heard that Apple’s new operating system, OS X Leopard, will support resolution independence, but I haven’t seen anything recently online about whether this feature is still supported.
However, I found confirmation of resolution independence in Leopard in a description of one of Apple’s World Wide Developers Conference sessions:
If your application uses custom controls or artwork, make this session a top priority. Modern displays vary in size and pixels per inch. Find out how to design a rich, scalable user interface for your application. The session will discuss guidelines for revising icons and artwork, new functions to adopt, testing strategies, performance concerns, as well as common problems and solutions.
There is also a video introducing the topic on iTunes here (Apple Developer Connection account needed).