Thanks, UpgradeMe, I was wrong on the LAX-MSP run. It is only double miles, not triple.
I have searched and searched for the thread I was thinking of. Can't find it. But I do have a little more info from Mileage Run:
http://www.flyertalk.com/forum/showthread.php?t=310651
http://www.flyertalk.com/forum/showthread.php?t=319206
Also, from the nwa site:
LAX/DEN Triple (may be EQM, depending on what your screen says when you register)
3) Qualifying travel must occur after registration and be completed not later than June 30, 2004. Qualifying travel must be on a paid published fare. Travel must be on non-stop flights between Los Angeles and Denver on Northwest operated flights 1-1999 and must make-up entire itinerary. Travel on flights between Los Angeles and Denver that include a connection from another flight do not qualify for this promotion. Example, MSP-DEN-LAX itinerary does not qualify.
Clearly, in the thread UpgradeMe pointed us to, the interpretation by the NW CSR is that a connection to any flight that day disqualified the promo. But that is an interpretation of the rule (and they ARE the ultimate interpreter of the rules!). BUT the rules for this promo don't say that. They refer to a connection on this itinerary.
Again, IIRC (which I can't prove), the LAX-MSP Double Mile promo says in the promo rules the RT has to be non-stop, LAX-MSP only, and then goes on to say no other flights on any plane for 4 hours either side.
I would honor the rules, but I think the CSR interpretation for the LAX-DEN rules are really applying to the LAX-MSP offer, and are not how the computer is treating the LAX-DEN offer.
Read the rules of your promo carefully. If you want to be entirely safe, send your own email request for interpretation in to NW, and use it as your private letter ruling to try to get what you want.