I had never though about this until I read the "How do I Unthaw yard hydrant underground?" thread.
In my shop I have a standard yard hydrant that feeds the sink, stool, etc. The hydrant is only about 6" from the wall so the underground portion is very close to the footing. I keep the temperature in the shop at 46 degrees F unless I am out there (which is rare in the winter) and then I turn it up to about 68. In short, the hydrant is normally in an area that is 46 degrees F and it is always turned on so there is water in the standpipe at all times. Also, it might go at least a week without any flow through it (stool not flushed, sink not on, etc.) so it does not freeze because water is moving through it.
It is hard to believe that the simple fact that it is in a building with limited heat would keep the pipe underground warm enough to keep it from freezing but it has not froze in the six years since I build the shop.
Oh yeah, I live in SE Iowa so it gets down to zero sometimes (like two nights ago).
Comments welcome.
Don