NW gives you the difference + $50 just because the price dropped???

Continental has some guarantees that the price on their website right now is the lowest available right now from anywhere (i.e. you won't find a lower price on Expedia or Travelocity). But I can't imagine any airline paying you back just because prices change over time (perhaps down).

But if you can confirm this about NW, I'm very interested to hear!