This morning, we got up (late - I slept like the dead last night) and went about our business. I started coffee, we computed, and so on. Then I went to go turn on the hot water, and... nothing came out at all.
This is where I note that it was 15 degrees when we got up this morning.
So, we thought at first that it was just the hot water heater that was frozen (we have a tankless, affixed to the outside of the house), but eventually, cold water stopped coming out as well, and we realized we'd been running off of the reserve tank out in the well house. Some troubleshooting later, we determined that it didn't seem that the pipes were frozen, and that something in the well house was - either the well pump motor, or something. So we put a light out there, and came in and poked at the internets, who suggested also putting a small electric heater out there. Eureka - 10 minutes later, we had cold water. (The hot water heater took until mid-afternoon to thaw; no, none of us own hair dryers.)
So we'll run a light overnight in the well house, and hope that keeps it warm enough out there. In the meantime, the water heater now sports a lovely new insulating box (Jeff designed and did most of the building; I held bits and pieces, nailed a few things, marked lines, and stuff). We can hook up a light in the box too, but we're hoping not to need to. I reckon we'll see in the morning.