There are biometric (fingerprint) luggage lockers inside security at the skyway concourse checkpoint and at at the entrance to the F concourse (next to the mailboxes), however I believe the limit for storing things there is 48 hours. More info. on these lockers and the complete list of locations can be found here:
http://www.mspairport.com/msp/airpor...s/default.aspx

You could always check with the NWA baggage office on the baggage claim level. They may not be set up for voluntary luggage storage, though. I suppose you could just check the bag and not pick it up for a week and see if they still have it when you get back to MSP.

One other alternative might be to check with the MSP airport lost and found office. Perhaps you could work out a deal on something? For more info on the airport lost and found, read this:
http://www.mspairport.com/msp/lost_a...d/default.aspx