I think Delta should at least match it's Skyteam partner fares. I fly ATL-MSP almost every week, and I usually have a choice of three airlines that fly direct, Airtran, DL, and NW. Since I live in Atlanta and prefer Delta's service, I choose to earn all my miles and status on DL, which I can do by flying either DL or NW. I just booked a few RT for May, and I was surprised that DL is not matching Airtran and NW's sale fares. Both Airtran and NW roundtrips were just under $200, and the cheapest DL at similar times was $250.

Of course, I choose the NW flight ... I still get my DL miles and MQM's, for $50 less. Maybe I'll miss out on a bag of Sun Chips ... big deal.