http://www.delta.com/content/www/en_...tificates.html

I stand corrected. DL.com says you have to use an AmEx card.

..... Restrictions:
Redemption is available only on delta.com. The certificate cannot be combined with another offer. Both passengers must be booked at the same time, in the same class of service and in the same record. Seats are only available in L, U, T classes of service and may not be available on all flights or markets. Most fares require an advance purchase of up to 14 days, a 3 night minimum stay and a 30-day maximum stay. All fare rules, restrictions, advance purchase requirements, and availability are per rule of Primary Ticket fare purchased. Companion tickets are not transferable. Primary Ticket and Companion ticket must be purchased with your Delta SkyMiles Credit Card from American Express. Both passengers must be booked on the same flights and dates, at the same time. Travel for Companion certificate must be booked and completed by the date on the front of certificate. Validity is not based on calendar year. Primary ticket will accrue miles. Companion ticket will not accrue miles. Tickets are not eligible for mileage upgrades, Systemwide Upgrade Certificates, or Medallion Complimentary upgrades. Companion Certificate is void if reproduced, altered, counterfeited, obtained or used improperly, or where prohibited by law. Companion Certificate has no value except when redeemed in accordance with all terms and conditions of this offer. Companion Certificate is for new purchases only and cannot be applied to the cost of prepaid tickets, ticket by mail, miscellaneous charge orders, Airscript or previously issued tickets. Companion Certificate will be deemed fully used once tickets have been issued and will not be returned or replaced. Companion Certificate is the property of Delta Air Lines and must be surrendered or returned upon request.
Fares, fees, schedules, offers and rules are subject to change without notice. Terms and conditions of this offer are as written and cannot be altered, modified or waived unless authorized by a corporate officer of Delta Air Lines, Inc. All Delta SkyMiles program rules apply. To review the rules, please visit delta.com/memberguide. ......