Wow, jessie, you were right! I just called again and got the woman I usually get (I recognize her voice, and she's not who I spoke with on Monday). And she said that no letter has been mailed to me, and no final decision has been made. She said that we will all be hearing from them within the next few weeks.
So, just go by the information I had before: If you are getting an interview, you'll get an email by the end of this month. Otherwise, it's a rejection. I asked, and she said she didn't know when they'd actually be sending the letters out.
I'd convinced myself that NU just wasn't meant to be, and that the rejection was for the best given their reputed gunner/competitive culture. Hmm.