Copyscape – Website Plagiarism Search – Web Site Content Copyright Protection: I think this has already been done. It’s called Google.
Copyscape is dedicated to defending your rights online, helping you fight against online plagiarism and content theft. Copyscape finds sites that have copied your content without permission, as well as those that have quoted you.
Speaking of Google, the copyscape search interface looks eerily like Google. Maybe Google should use the service. Maybe Google is the service.