You might not get a whole lot of replies on this, because I don't think there are any NW pilots on this forum, and no employees (maybe one or two, I'm not sure if A330_DTW or A32 work for the company).
That said, I haven't heard anything about MSP-JAX, but if NW were to have started JAX service, my guess is that they would have done it before now to catch the seasonal growth to JAX (granted, it's not MIA or RSW, but it does get some seasonal traffic).
In the past, Northwest has started routes from secondary cities if people start flying routes to the secondary city connecting through the primary city. That may sound confusing, but it's easier said with examples...like MEM-BWI, a route which was started about two years ago. Enough people were flying MEM-DTW-BWI that NW started that route. Other examples include (the short lived) DTW-FCA, MSP-FLL, and MEM-PHX (recently restarted after the route was suspended three years ago).
My point is that if you want JAX-MSP nonstop service, a good idea would be to book one of your trips to MSP on NW through DTW or MEM, rather than JAX-CLT-MSP or JAX-ATL-MDW-MSP, on USAirways and AirTran respectively (your last two routings to Minneapolis, if I'm not mistaken).
Good luck with the route! I hope you in JAX and us in MSP get the flight as well.
Create your own luck.