I have me read David McKay's book, and met him a couple of times as a result of doing an MSc in Environmental Technology at Imperial, below is what i know to date
Heat pumps are a currently receiving a great promotion in the UK (already well established in Japan), and even more in April 2011 with the renewable heat incentive which will give you money for running a heat pump - unlikely to be more than running cost sadly ;)
A heat pump can be air source, or ground source. they are in effect a refrigerator in reverse (moving heat from out side your home to inside - like a fridge moving heat from inside the fridge to outside)
although they can be 500% efficient, the reality is that they are likely to operate between 250-350% considering UK weather, complicated further is the temperature they yield = about 50 C - good for having a bath, not so good for central heating (pushing a heat pump to supply +60C will reduce the efficiency enormously). the reality is that heat pumps work best with underfloor heating - low temperature, near constant applied heat.
so they can be used in beautiful Victorian houses, but be prepared to sacrifice efficiency for your original tiled and floor boarded floors - as well as a large installation bill (returning to a hot water tank)
I am currently supervising my father's installation of a 15kW Mitsubishi Air source Heat pump on the Isle of Wight - but live in Kensal Rise.
With a de-carbonised electric grid, heat pumps will be the lowest carbon heating solution, but we're still a way off that.
hope this helps
Really interesting to read your thoughts. Thanks very much for that. Hopefully we can meet up face to face at some point and chat about things, particular the government's 'Roadmap to 2050' due out this month I believe and which will presumably be heavily influenced by David MacKay's thinking.
What transition events are you involved with locally?