If you are thinking about the outdoor plumbing thing, PLEASE do a drain-back system of some kind. The small upfront expense of pumps and thermostats and such is so much less than busted, frozen, or melted pipes down the line.
Look here:
Solar Water Heating Projects and Plans
If you can eliminate the water from the outdoor collector, that would be ideal as far as reliability goes. Here's an idea:
Run some PV panels to the electric element in your geospring, or do as xringer has, and go buy a cheap(er) electric resistance heater that runs straight off the PV panels. When the sun is shining and your preheat tank is good, feed your extra power back to the grid through a micro-inverter.
He has been doing this for awhile now, and previously owned all manner of water-heating apparatus. Works like magic comparatively.
http://ecorenovator.org/forum/solar-...html#post29754