I just got this email from Delta, and Northwest seems to have a similar offer out there at the moment:
Dear Mr. Lucky, Book Now!
You now have 500 reasons to buy your next ticket at delta.com.
Earn up to 500 bonus miles on all flights you book at delta.com between April 28 and May 31, 2009. No enrollment is required!
Here are the full terms and conditions:
Terms & Conditions
Eligibility: Promotional enrollment is not required. To participate in this offer, eligible members must have a SkyMiles® account with a U.S. address. Bonus Offer: Earn 500 bonus miles for each flown round-trip qualifying ticket (250 bonus miles for each one way ticket) purchased at delta.com. A qualifying ticket includes any Delta or Northwest coded flight purchased at delta.com. Flights must be sold under the DL or NW code. Bonus miles do not count toward Medallion® or Million Miler™ status. Bonus applies only to the SkyMiles member flying and purchasing the qualifying ticket at delta.com. Purchase Period: Tickets must be purchased between April 28, 2009 and May 31, 2009 for travel anytime. Best Fare Guarantee: Best Fare Guarantee is valid for electronic tickets purchased by credit card with a United States billing address at delta.com for travel originating in the 50 United States. Voucher offer only applies for lower published Delta fares found that same day for the exact same itinerary including the exact same Delta flight, cabin, booking class and flight time of the itinerary purchased at delta.com. Difference in total fares (including third party booking fees) must be equal to or greater than $10. Cancellation/refund request must be made by midnight the first day of travel or midnight the day after purchase is made, whichever comes first. For a voucher claim to be valid, it must satisfy all terms and conditions and be submitted in compliance with the claims processing procedures. Fares, offers and rules are subject to change without notice. Additional restrictions apply. Visit delta.com/bestfare for additional details Additional Restrictions: Bonus miles will post to the SkyMiles account of the qualifying member 4-6 weeks following the end of the promotion (May 31, 2009). Taxes and fees for Award Travel are the responsibility of the passenger and must be paid at the time the ticket is booked. Award Travel seats are limited and may not be available on all flights or in all markets. All SkyMiles program rules apply. To review the rules, please visit Membership Guide & Program Rules. Offer void where prohibited by law. Other restrictions may apply. Offer subject to change. Please refer Delta customer service representative to 970639.
While it’s not a big enough incentive to book Delta if you otherwise wouldn’t have, it’s a nice bonus.