There's a difference between upgrades being served by an airline out of desperation (covering ...) and out of goodwill.
In your case (not to rain on your parade), it was offered out of desperation.
Think of the alternative scenario: "Oh, Mr./Ms. Letstalk, we're completely overbooked. Can I give you a $300 voucher if you accept delaying your embarking on this upcoming 12 hour flight?"
If there truly were no seats available for you on that plane that day, they'd certainly let you know.
How would you feel about NWA then?
[This message has been edited by FQTV (edited 06-24-2000).]