If you're REALLY really concerned (and I don't think there's reason to be, like Russ said, there's lots of other things you should be worried about before radiation from cellphones), you could build yourself a http://en.wikipedia.org/wiki/Faraday_cage" around your house. A Faraday cage is the reason you can see through the window on your microwave, instead of it being a solid sheet of metal.
BUT, enclosing the walls of your house with a Faraday cage would be obscenely expensive after the house has already been built, having to remove drywall, install the cage, re-install the drywall, paint, etc...