Yeah danopia, I saw that rereading through the archives again. I think actually Dilbert mentions it on 02/03/1991. But in my defense, the Sunday strips were not posted that far back on this site back when I made my original comments. And I thought I came up with a great original idea. It burst my bubble.
Burning junk mail for heat seems smart, but don't forget that there would be a lot of pollutants given off, so I'd want a really good incinerator, or a really good dispersal for the resultant gasses. Maybe to PHB's neighbourhood ...