Energy Star says you need to use the desuperheater or an integrated on-demand hot water heater to get the tax credit. Because I am interested in a heat pump water heater with tank (HP-50) it would stand to reason that it would be more efficient to just buy an on-demand heat pump unit that runs off the wells. Does anyone make these things? I'm buying Climatemaster 27 but I can't find such a unit on any of the popular brands.

If you are asking if any of the brands have on demand hot water the answer is yes. Water furnace, Hydron, Hydro Delta and I think Econar does as well as about any DX system.
That said, while I can't speak for the track record of the first two (don't know how long they've been making them off the top of my head) the third is the source of many, many complaints.
My personnal feeling is that I am not a fan of using the most expensive appliance for doing the job of the cheapest.
Joe

A high end solution that avoids both Joe's concern with excessive cycling of the main heat pump as well as potentially delivering superb COPs (anywhere from 3 - 6) is to add a dedicated water to water heat pump for hot water. WaterFurnace's EW020 is an example. It has a double wall vented heat exchanger for safety of potable water.

I haven't priced one of these but installed cost probably runs to several thousand dollars, so it would probably make sense only for big families needing lots of hot water with a long ROI horizon.

Connected to a geo closed loop it has the additional benefit of pulling heat from the loop during cooling season, although that could become a liability in winter.

OK. I figured there was a reason everyone here was suggesting a desuperheater and unpowered tank. Probably go with two 50 gallon Kenmore 12 tanks (R24 ins., 12-year warranty) and only power the second tank when I have company. Thanks for the help.

Energy Star just sent a draft to all Energy Star partners about this. Its wording removes the requirement for the geothermal to do any of the domestic water heating to be Energy Star rated. Instead, it requires that manufacturers "offer" it as an option. It's expected to go into effect in October...maybe latter Oct....

Thanks for the update. If I plan on not selling for a long time I would assume that the desuperheater is worth it anyway. I'm electric water heat.