Stephen Hood has the post on the Yahoo Search Blog announcing their new feature Instant Search.
It essentially takes the shortcuts that Yahoo and others provide on a web search and displays them before you even have hit the search button.
It's a cute idea, a nifty little AJAX feature that, like Google Suggest, might provide useful interactivity to normally static web pages.
Unfortunately, the algorithms seem to need a little work. I would think the answers should pop up in any case where Yahoo is sure they have the answer (so it's worth the interruption of a popup) and not in any other cases.
But I found the popup coming up in cases where it wasn't useful (a popup with news on "weather" when I typed "weather" and a popup for news on "capital" when I typed "capital") and not coming up in cases where it should (no popup for "capital washington state").
Despite the flaws, you have to admire the ambition of this move toward perfect search. Ideally, a search engine wouldn't show you a list of results at all, but just the information you need. Clever innovations like this one may be steps toward that goal.
See also comments and reviews from John Battelle, Danny Sullivan, and Charlene Li.