Originally Posted by GBadger

Yes, that would be allowed. BNA, PIT, STL and RDU are all part of this promotion. So, any trip that STARTS and ENDS in those cities should be fine. There have been some problems reported with people that do things like:
DTW-MSP-BNA/short break but separate tickets/BNA-MSP-XXX-MSP-BNA//BNA-MSP-DTW

I think that was due to the connections taking the person through MSP twice in close succession.