From an email received from NW:

Northwest is rewarding our WorldPerks(R) members with up to 1,000 WorldPerks Bonus Miles for giving us their email address.
If you are not a WorldPerks member, register online at http://www.nwa.com/freqfly/direct/enrol/ and then register your email address and WorldPerks number by visiting http://www.nwa.com/wpemail

If you are already a WorldPerks member, make sure we have your WorldPerks number by going to http://www.nwa.com/wpemail and updating your registration information with your WorldPerks number in order to receive the Bonus Miles.

From the Web site:

If you are already registered to receive email from Northwest, you may qualify for the promotion by updating your email registration and providing their WorldPerks number through the registration form below.

[This message has been edited by skyflyer (edited 03-15-2000).]