Apple's got it right.
First off, this is probably best left as a convention as opposed to something deep in the OS. But the key to making it work is to provide clear documentation and several applications which use your protocol, or what have you. If every Mac has the iLife suite, and every iLife app is relationally enabled, then you have an existing platform. As has been said before, the one thing MicroSoft has shown above anything else is that a platform beats an application every time.