I thought medallions were allowed to check overweight bags, up to 70lbs.

"Can Medallion members continue to check bags at no charge?
Medallion members traveling on U.S. domestic flights in the Economy cabin are permitted to check two bags up to 70 lbs. each free of charge. (International flights allow 2 bags up to 50 lbs.)"

From: http://www.delta.com/help/faqs/bagga....jsp#medallion

EDIT: I should add that whenever I travel for leisure my wife always breaks the 50lb mark. We've never once been hassled for an extra fee during check-in.