Mac OS X has implemented a lot of clever things in this area for a long time. There's the obvious case of user applications being self-contained bundles, which

Mac OS X has implemented a lot of clever things in this area for a long time. There's the obvious case of user applications being self-contained bundles, which works because nobody ever links against anything not provided with the system by Apple. The really interesting part is the operating system ships with multiple independent versions of the SDK libraries, and the developer tools ship with several independent sets of headers for them. An application that was built against the 10.4u or 10.5 SDK will run against those libraries even on 10.6.