I have recently been building a predictive Search engine for a project for Canon to be released at this years Drupa in Germany. I cant release the code just yet but i will as soon as the project has gone live.
You can see it in action here you can search the ( very small ) archives of posts by typing in the search area. The predictive search is performed on the Titles of the Posts whereas if you press return or click search a full search is performed on both the titles and their content. Try it out 'Predictive' is a good place to start!.