The only offer I've seen recently is a 25% bonus that you have to sign up for. It also contains some info. about reward caps. Here's the details:
Earn 25% More Miles On All Purchases from October 1 - November 30, 2006
Enroll Now!
Enroll today and use your Delta SkyMiles® Credit Card from American Express in October and November and earn 25% more miles on all eligible purchases. That's 25% more miles on all purchases from appliances to zoom lenses.
For additional information, please see the Terms & Conditions below.
To be eligible for this promotion, you must enroll using your Delta SkyMiles Credit Card account number (the 15-digit number on your Card) by November 30, 2006. Please have your Card available when you enroll, as you will also need your Card ID number, the 4-digit number printed above your account number on the Card.
By clicking the "Enroll" button, I certify that I have read
and agree to the terms and conditions below.
TERMS AND CONDITIONS:
Enrollment required by 11/30/06. If you prefer to enroll by phone, please call 1-800-653-6051. The 25% bonus will be based on the total dollars of eligible spend posted to your account from 10/01/06 to 11/30/06. Please allow 4-8 weeks after the promotion for the bonus miles to be posted to your Delta SkyMiles account. This offer is available to Delta SkyMiles Credit Cardmembers only. Cardmembers who earn Always Double Miles® will continue to earn Double Miles for eligible purchases. To be eligible to receive bonus miles, Cardmember's account must be active and not in default at time of bonus fulfillment. Balance transfers, Express Cash, and U.S. Savings Bonds transactions do not apply. Bonus miles for this offer are limited to 20,000 miles. In each year of Cardmembership, your mileage earnings are limited to $60,000 of eligible spending ($100,000 for Gold Delta SkyMiles Credit Cardmembers, unlimited for Platinum Delta SkyMiles Credit Cardmembers), not including bonuses. Promo Code H460:0001. All SkyMiles program rules apply. To review the rules, visit delta.com/skymiles. ©2006 American Express Bank, FSB All rights reserved