Continental Airlines inflight currency, the kind that pays for drinks and headsets, is acceptable onboard by Northwest. I never realized that, and apparently many Northwest flight attendants don’t, either. (In addition to selling scrip at checkin kiosks, Continental gives these out to their OnePass Platinum members.)
It’s confirmed by Northwest on a Flyertalk thread that the Northwest flight attendant manual is clear on this fact under amenity coupons (section 365.1.4 if feeling particularly confrontational about the subject).
A very minor issue, to be sure, but something I never knew for sure. Drink up!