I was curious, so I checked on this...

From http://www.nwa.com/features/bestfare2.html

"When it comes to finding the best Northwest Airlines' fare online, you can count on nwa.com. If you find a lower fare online for the same flight, itinerary and cabin that's at least $5.00 lower on the same day after you purchase your nwa.com ticket, file a claim and we'll refund the difference and give you an Electronic Credit Voucher (ECV) for future travel in the amount of $50.00 USD for each eligible ticket."

So, just like CO, this is a guarantee that their website is the cheapest place to buy a ticket. It is NOT a guarantee that you are protected against future price drops.