Been around for a long time bro!
Hot water add-on type systems have been around for ages on wood, coal, oil etc. etc.. My moms old Monarch kitchen range (coal, wood, sawdust burning) had a heating coil (sort of like a coil spring but bigger) in and over the firebox, there was a two tank system (I think one recirculated, one was a pressurized electric hot water tank) and it not only helped heat a big ol' house but provided massive amounts of hot water when running the stove.
Tons of info out there for various systems and if you shop around and DIY it doesn't break the bank and the over-all savings can be huge. Years ago I had an idea to mount a convection heating system to a stovepipe to take advantage of wasted radiant heat loss, basically a coil going around the outside of the pipe in the attic of a house I lived in, never did it, but the technology is old. Do an hour or two of googlin' "stove heated hot water" or "wood-fired hot water systems" and you'll see what I mean, plenty of forums and free info out there.
Here is a link to Popular Mech.'s forum on this very topic which should lead you to some good stuff (great site too for many tech Q's and the curious minded).
Here>
http://www.popularmechanics.com/blogs/energy_family_news/4204882.html
Personally I'd like to see more heating setups take advantage of the wasted exhaust from wood or oil heating, seems a shame to have it all go up with the smoke
The answers are out there however, and very accessible.
I'm cruising the boards right now but am curious about this so if I find any interesting links I'll be sure to post them here.
And let us know whatcha find too eh!]
Serge