Idea for a mashup - now, to make it happen...
kiwidrew: Unfortunately, you're probably going to need to take elevation and topography into account to get anything useful.  As far as I know, none of the web mapping APIs really expose that data right now.  (Google Earth may, I don't have much experience with it.)Here in Wellington (NZ), the local terrain is quite hilly, and as a result the amount of sun a house gets is a big factor in purchasing (or even just renting).  Especially in wintertime, when the sun is often so low in the sky that many houses, streets, and even neighbourhoods miss out on the sun entirely!  (At my house, in about a month we go through a 2+ month period of no sunshine, which takes some getting used to.)So of course someone built a website to generate 'sunshine reports' for any given address!  I'd recommend checking it out.  They use topographic data to (roughly) estimate how much sun you'd get during each month of the year, complete with pretty charts and graphs.  Still doesn't take into account trees and other buildings, but (as far as I know) that data simply isn't available at all.http://www.keyring.co.nz/intro/index.jsf