The latest version of Google's search application for the iPhone lets users enter search terms simply by speaking them. Voice search poses a big challenge: where an airline reservation system, say, can make do with a limited vocabulary--dates, destinations, and the like--search is totally unconstrained. But Google has taken advantage of its wealth of data on how people search: if the application has trouble interpreting a word, it can consult tables of terms that are frequently used together.

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