Why not call Northwest? They should know their own policy...
We comped last December and got through 2004 (will requalify honestly next month through 2005, folks).
A thread over on Delta says a flyer there was just comped through 2004 also. (He expects to make enough between now and then to earn it honestly, too).
So evidence is sometime between July and December. As I said, why not call and ask? It won't cost you anything.