It's the 50% bonus for "Premium" (YBM) tickets:

8. ** 7/12/2009 DL double mile bonus (200% instead of 150%; Still 150% EQM) for First / Business Elite / YBM fares 7/7/2009 thru 9/7/2009: https://www.delta.com/marketing/doublemiles0709/ -- and Yes, it is TARGETED ONLY.

T&C:
Bonus Offer:
Purchase and fly round-trip on Delta or Northwest marketed/operated flights between July 7 and September 7, 2009, and earn a 50% mileage
bonus for each round trip in First Class (F and A fare classes on Delta marketed flights and P and F on Northwest marketed flights),
BusinessElite/Business (J, C, D, S, and I fare classes on Delta marketed flights and J, C, I, and Z on Northwest marketed flights), or Economy (Y,
B, and M fare classes on Delta marketed flights and Y, B, and M on Northwest marketed flights). Delta and Northwest marketed codeshare flights
are excluded from this offer. Bonus miles do not count toward Medallion® qualification or Million Miler™ status. Paid fare classes H, Q, K, L, U, and
T on Delta marketed flights and paid fare classes H, Q, V, L, T, K, on Northwest marketed flights are not eligible for a bonus with this promotion.

Travel Period:
Travel must be between July 7 and September 7, 2009. All travel must be completed by September 7, 2009.