Despite widespread denial by Apple's fans, MacOS (currently 10.6) has no working support for resolution independence, and plans in this regard have quietly disappeared since Leopard. The ruins remain, however, and can still be activated in the command line. Most of Apples's own applications (especially Finder, Mail, iTunes) will display badly ruptured interfaces as soon as the DPI setting is changed. Apple's IDE (XCode) will even fatally crash, indicating that Apple's developers are currently not working on the problem.
Because implementing resolution independence will require not only the adaptation of the OS and Apple's own applications, but also 3rd party applications, Apple will likely give about 9-12 months notice of plans in that regard. So: probably no resolution independence in Lion either. But perhaps we get workarounds (configurable font sizes within individual applications).
The problem is very bad and frustrating already, especially with the advent of high resolution screens in the 17in MBP and 27in Cinema display, which many folks find unusable.
In Windows, resolution independence has been working since XP (although the settings are somewhat hidden). Some of icons will scale badly, and in the early days, some applications had text and buttons larger than would fit into the dialog boxes, but these problems are almost gone. For most applications, Windows can be used with arbitrarily high resolutions.
/Developer/Applications/Performance Tools/Quartz Debug.appwhich is fun to try out (but nothing more). – Daniel Beck Oct 7 '10 at 19:43