I agree with PARSpro that codeshares don't reduce competition. Throughout most of the 1990s, Delta was Northwest's primary competition (in terms of agreesive fare pricing) in Detroit. But then Spirit and Southwest became NW's primary competition, NW started matching their pricing and DL dropped out. Then Continental became agressive in the Detroit market, and stayed that way until March-April this year. Now they're among the most expensive.

I personally don't think NW will adapt "insULT" fares a la CO/DL; Richard Anderson is smart enough to realize that most of his premium flyers choose his airline because of favorable upgrade policies. NW would also be in position to attract Delta flyers. That said, it wouldn't surprise me if NW added an addendum to their upgrade policy: "COMPLIMENTARY UPGRADES FOR ALL ELITES: excludes flyers in Michigan and Minnesota."