The head gasket is covered to 100k miles by Subaru as long as the car has had all its service bulletins taken care of. Basically all Subaru does to stop the leak is to put Stop Leak in the coolant. This is their "conditioner" and their fix for this common issue.
Because of this, I would of course suggest that after any anti-freeze change, buy a bottle of Subaru's conditioner to toss in. It's kind of a dumb fix, but if it keeps customers and Subaru happy, so be it. If anything does go bad, at least you had the conditioner in and they can't whine about that.
I've heard that people step to STI head gaskets and prevent the problem too.