The e-cert PD-925 mentioned is only one of the Northwest $ off coupons from amenity kits that people receive when a flight is delayed.

There are a number of other ones as well PD924, PD886, PD964, etc with different expiration dates and discount amount.

PD924 fixed $35 off $200+
PD925 discount amount see above, expires on 09/2005
PD886 same discount as PD925, expires on 06/2005
PD964 same discount as PD925, expires on 07/2006

You can find more information from this thread
http://www.flyertalk.com/forum/showthread.php?t=420383