Looks like it's a response to US Airways' new DCA-DTW nonstops. Interesting difference is that the US Airways double miles are EQMs, but the NW miles are bonus miles. From US Airways' website:


Increase Your Capital via the Capitol --
Earn Double Miles Towards Preferred Status

Introducing Nonstop Service between Washington, D.C. and Atlanta, Chicago O'Hare, Cleveland, Dallas / Ft. Worth, Detroit and Houston

Dividend Miles members will earn double miles good towards Preferred status with the purchase of any paid fare for travel on nonstop flights between Reagan Washington National and Atlanta, Chicago O’Hare, Cleveland, Dallas/Ft. Worth, Detroit and Houston through June 30, 2005.

To qualify, register one time before your next flight by completing the form below.

New Service Details
No airline knows Washington, D.C. like US Airways – it’s our hometown. Now, we are expanding even more to fit your needs. Beginning February 6, 2006, we are introducing new nonstop flights between Ronald Reagan Washington National Airport, located near the heart of the city, and Atlanta, Chicago O’Hare, Cleveland, Dallas/Ft. Worth, Detroit and Houston.
We have scheduled our flights to be timed perfectly for same-day business trips. New Nonstop Flight Schedule Between Washington, D.C., beginning February 7, 2005:

New Nonstop Destination from Washington, D.C. Daily Flights
(Monday - Friday)
Atlanta 4
Chicago O’Hare 4
Cleveland 4
Dallas/Ft. Worth 3
Detroit 4
Houston 3
Some service only operates Monday through Friday. Schedules subject to change with or without notice.

Travel in Comfort
Service on these nonstop routes includes new Embraer 170 jet service with all leather seating with adjustable headrests, no middle seats, wider seat width and generous legroom, lots of headroom and ample overhead bin space.

Now you don’t have to split your travel among several airlines to or from D.C. You can bank on US Airways and Dividend Miles. US Airways, US Airways Shuttle and US Airways Express offer nonstop service to almost 50 destinations from Washington (DCA), with service connections to over 200 cities throughout the U.S., Canada, the Caribbean, Latin America and Europe. With the Star Alliance network and our many partners, you can earn and redeem Dividend Miles to over 770 destinations in 133 countries around the world.

We hope to see you onboard another US Airways flight soon to Washington, D.C. or to any of our other worldwide destinations.

Terms and Conditions: To qualify for bonus miles, one-time registration is required prior to first flight. Double miles bonus valid with the purchase of paid fares on US Airways or the US Airways Express carriers between Reagan Washington National Airport and Atlanta, Chicago O’Hare, Dallas/Ft. Worth, Detroit, Cleveland and Houston, February 6, 2005 through June 30, 2005. All or part of the service is offered on regional aircraft operated by US Airways Express carrier, PSA. Not valid on codeshare flights operated by airline partners, such as United®. Offer can be combined with any other promotional flight bonus or other offer. Retroactive mileage will not be given prior to registration. All Dividend Miles terms and conditions apply.