Welp. I had a friendly chat with an Apple rep on Tuesday who assured me that I was still on schedule to receive my iMac today (Wednesday, 10/21) even though it hadn't changed to "Shipped" yet. Sure enough, at 5:41am local time today, I receive an email from Apple with "Due to an unexpected delay..."
tl;dr, I should now expect delivery 10/23-24. Oh well, at least my Nexus 5x arrives today.
Oh, BTW:
4.0GHz QC i7, TB up to 4.2GHz
32GB 1867MHz DDR3L (4x8GB)
256GB Flash Storage
AMD Radeon R9 M395X w/4GB
Magic Trackpad
Still "Processing"