I had a discussion with an engineer with Dandelion Energy, which manufactures geothermal equipment. My house presents some complications which make it not cost effective, mostly related to the fact that the house was misengineered from the outset. But I can see where this could work for many suburban homeowners.

The heat source for the system is a well. The depth of the well would depend on the heat requirement of the home and the local geology. The transfer fluid is a water/glycol mixture. The wellhead and connecting plumbing are all below grade, so plant some grass and nobody will see anything. The question of neighbors: the well is obviously vertical, 500 feet deep in my case. As heat is transferred between the ground and the home, a "cylinder of disturbance" will form around the well as the season progresses. But it's only about ten feet in diameter, so unless you are very close to a property line, there will be no problem with neighbors.

A heat pump in your boiler room transfers heat between well-moderated water and R422 in either heating or cooling mode. This is then pumped through a conventional forced air system. There still needs to be a domestic hot water system, electric or whatever. And probably some sort of power backup.

Cost is initially high, because it's still early in the tech cycle and because well drilling is costly. Compared to oil heat with conventional A/C, your energy costs would be roughly half, so there will be ample cost recovery over time. There are currently a lot of private and government incentive programs (my utility offers $5000, and NYS has tax credits). Eventually economies of scale will make the cost of these systems comparable to an A/C system, plus the cost of a simple well. It seems that it's most cost effective to design a home around geothermal from day one, rather than to retrofit. Retrofitting cost would be cheapest in a house which has a single air handler which handles both heat and cooling. (This isn't the case for me, which is why it's not cost effective.) I think that if you're in a situation where you're dealing with new construction or a simple existing house, this can be a very good way to go. If I was building a new house, it would do it without a second thought.