Originally Posted by
ecaarch
FTraveler has it correct. Buy the ticket from DL with a DL flight number and you will always be safe (until somebody decides to "enhance" it.) It is possible for two DL pax on the same flight, each crediting their mileage to SkyMiles, to earn differing amounts of SkyMiles/MQMs based on whether the ticket was purchased from DL or from the operating codeshare partner.
The deciding factor is the DL flight number on the ticket, whether you buy from Delta or from any other vendor such as Expedia.