Well, my trip to a local photo shop was very productive. They offered to take a look at it for me and get it sent in to Nikon for repair if necessary. The guy took one look through the view finder, then took off the lens to confirm that the mirror was in fact stuck in the up position. As we were talking about what to do next, he gently touched the edge of the mirror and down it came. He was able to fire off about 20 shots, turn the camera off and back on, then fire another 20 without issue. So for now, it's working. I suspect I may have this problem again, but since I'm not under warranty, I'm in no rush to get it sent in.