I don't have any factual citations, but I suspect they're a lot more common than you think. At the very least, they're completely ubiquitous in central Texas. I'd imagine that's a combination of temperate conditions (as opposed to Minnesota, it never gets to the 6F temperature here), and newer construction (older houses are probably less likely to have a heat pump anyway, but with the insane growth in real estate around here, there's not very many old houses).