Thats great that folks are thinking of putting up bat houses. Bat Conservation International is a great organization. Bats often get a bad rap, but are an important part of the ecosystem. I think BCI has some guidelines for locating the houses and what color to paint the outside, based on what latitude you live in. This helps with temperature control so they don't get overheated. I would put some houses up myself, but I am just a tad to far north.
Jonathan