I just finished a trip yesterday and did it without any problems. My outbound was last Friday (even got upgraded DCA-DTW!) and I did online check-in (I've never tried switch at the gate) and entered my NW number before actually checking in. Yesterday I was upgraded all the way back to DCA (BNA-MEM, MEM-IND, IND-DCA) with my NW number on the boarding passes and Silver Elite noted on the pass.

I'm glad it worked because I was getting a little nervous after monitoring this thread. Luckily I'm just about to Silver and won't have to worry anymore!