Inspired by Apples new Spotlight feature, I implemented something similar using JavaScript with XMLHttpRequest and a little bit of PHP on the serverside. Just go to the search field on the right and start typing a search word, it should then immediately show the results below. It works on Mozilla, Safari and even on Win/MSIE.
Currently it only searches through the titles of all our posts, maybe I will extend that some day. And I have some more ideasā¦
Update: There's now a LiveSearch Roundup and a wiki page for the whole project. Please visit them, if you're further interested in the project.