This topic has fascinated me for some time.
There is only one local project that I know of that is doing this in West MI, and it is a commercial borefield. GMB an Engineering firm based out of Holland, MI does full service arch/eng work, but specializes in commercial geothermal. They partenered with a local commercial grower who uses geothermal to heat their greenhouses in Kalamazoo, MI (no cooling involved). In 2009 or 2010, they installed solar thermal panels to help "charge" their borefield. I haven't heard or seen anything on the results of the "solar charging" project.
As someone already noted, it would seem really easy for the entire "solar charging" project to go negative in the overall life cycle cost compared to just plan geothermal, once you account for additional pumping power, cost of the panels/piping, life of the solar panels, and additional maintenance.
I would think alot of field testing/bores and a solid modeling tool/software would be involved to know if this concept would payoff and how to best operate or control it, since there are so many variables (ground conduction, borefield size, collector size/type/allowable or achieveable temperatures, fluid flow rate/pump speed, weather/cloud cover, etc.)
Depending on the above variables, is there a point as the ground warms that the the heat flow into the field is less than the pump energy expended to circulate the fluid?
If this took place before the heating season starts, then the energy benefit is erroded over the time between that and the start of the heating system.
Finally, would putting the project cost of a "solar panel charging" into expanding the borefield have the same or better overall lifecycle cost ??
An intersting project that is using this concept is Drakes Landing in Central Canada. See there web page with live data here....
http://www.dlsc.ca/