I think I'm the only home in my neighborhood without geothermal heating and air conditioning, as our house is much older than the rest. There was one other hold-out from the mid-1980's, but he upgraded to geothermal about 5 years ago, and I was amazed at how inexpensive it really was. I guess the costs have really come down.
Despite being larger properties, I think all of the systems in this neighborhood are the deep well variety. There would be no issue with the shallow field type either, we're each on several acres, so there must be some cost advantage to going deep well on an existing property. These are larger homes (6000 - 13,000 sq.ft.), so each system has several deep wells.
But I do wonder what having a hundred million suburbanites on 1/4 or 1/2 acre lots, each with several deep wells for geothermal, will ultimately do to our ground water. Could the increased temperature caused by pumping all of that summer heat into the aquafers lead to undesirable biology? Could the subsequent cooling of the aquafer caused by pulling so much heat from it during the winter cause undesirable geological activity, due to thermal cycling?
One of the things that frustrates so many is industry and media so often pretending the next new solution, whatever it is, is going to be the answer to all our woes. "Oh, no... it's panacea, it doesn't create any new problems, only fixes the old ones." Short of a mass extinction event, there's nothing we can do that doesn't have impact on the environment in which we live. All we can do is push that impact around to the direction that hurts us least.
Many here have already brought up diversification, and that's surely part of the solution. Not just for reliability and financial reasons, but in terms of distributing the negative impacts of any one technology.