The correct solution is for the stupid operating system to realize that every stupid application stores defaults and shouldn't have to even think about this problem. We're talking about a trivial api, even if we're dealing with opaque data -- the stupid OS should decide, and the apps should write to that API. Even my own code doesn't decide things like this -- I write an API that handles picking file paths and opening file handles, and then all of my code uses that API. Boom. Done. I never think about paths again.
Apple has this. The command line program to manipulate it is called "defaults". The files it manipulates are stored in ~/Library/Preferences, but the application isn't supposed to care about that.
