I just noticed that Askville has an almost identical feature as Y! Answers. I don't remember if this is new, but it's remarkably similar.
It's the search / ask ahead widget that I worked on.
Also to note is that the plumbing is fairly similar as well. the XHR request fires after two (2) words. The response is markup in the form of an unordered list. However, there is no caching of the result set within the client, so for each keyup event fired (with aforementioned requirements), the XHR is sent; just a performance thing.
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