If you are Platinum Elite in KLM's
Flying Dutchman program, you get free upgrades on Northwest domestic flights (subject to availability, of course) on any fare. (Actually, I believe you may get them as a Gold Elite as well, depending on availability, but first crack goes to the Platinums.)
At least, that's the case through 31 December 2004. They haven't published the T&C for 2005 yet that I know of.
Problem is that in order to be a
Flying Dutchman member, you must have a non-North American (and some other areas) mailing address.
Also, you
don't get free upgrades on NW internationally, or on KLM at all

.