Interesting policy consideration. Would be interested to see the carbon impact calculation on this one..... http://www.bbc.co.uk/news/world-europe-20041522
Well to prevent freezing you don't need high temperature, so Geothermal (like in the article), or even waste industrial/power generation heat could be used. The real cost here is probably just the piping, and the corresponding maintenance that piping will take. And of course some very minimal (when you think about the scope of the project) pumping costs.
I've seen systems like this in Japan that simply uses heatpipes to conduct the deep heat (down beyond 10m?) up to the surface - even 5C would do the trick, and there's no carbon cost except for... the smelting of the copper?