I have used the system. It is pretty slick.

Read more: http://www.nwa.com/checkin/eboarding/

Note (from the FAQ section at the bottom): "E-Boarding Pass is currently offered on non-stop flights from Indianapolis, IN, Detroit, MI, and Minneapolis, MN during check-in on nwa.com. If you are departing from IND, DTW, or MSP you must be checking in as a single passenger on a Northwest operated non-stop flight within the United States."