That would violate their terms of use, and they would shut you down.
They have given some sites permission, such as http://housingmaps.com, as long as no ads are run.
Oodle tried it, and CL stopped them.
Yup. Short of an official supported API, we are pretty much tied on utilizing CL's data. While screen scraping CL is easy and can yield fully utilizable data for an outside application, the application will be doomed to failure (financially, at least) because it will always be arguable in court that your application is worthless and that it is their proprietary data that ultimately holds the value of your application.