Via NotiFlyer, and subsequent to my post on taking $50 off a $250 Southwest ticket using Paypal, it seems that Northwest is making a very similar offer.

$50 off $250 with Northwest when you pay with paypal, like the Southwest offer tickets must be purchased by March 27, 2008. This offer limits you to travel between April 1 and June 14, 2008, and Northwest’s change policies include fees so you can’t circumvent the travel period in a worthwhile way.

This one is more flexible with Southwest as I previously noted, but if you’re buying Northwest tickets anyway (and I wish I held had off on some myself, d’oh!) this is useful.