Update: This bonus has been extended until 04/30!
Originally posted on 02/01:
You can now transfer miles between Delta and Northwest in either direction for free, as many times as you want!
You need to link the accounts first, and you will get 500 miles just for linking the accounts!
Although the description of the offer implies that you need to actually transfer miles to get the miles, the terms make it clear that you only need to link the accounts.
“Members of the SkyMiles and WorldPerks programs may link their accounts and receive a one-time bonus of 500 bonus miles. To be eligible for this offer, the member will need to complete the linking process application on delta.com between January 29, 2009 and March 15, 2009. Bonus miles will post the member’s SkyMiles account 4-6 weeks following the end of the promotion, March 15, 2009.”