So the voice search is called Google Now? During the keynote it was confusing because they kept calling the voice stuff "Voice Search" and "Google Now" was only used when talking about the cards (which are confusingly in the same app).
Voice Search has been available since Froyo, IIRC -- although in the past it would simply show search results rather than respond with a voice for anything other than a small set of phrases. Voice responses for more general Knowledge Graph queries is new in Jellybean. (Long-press the search button on an older phone, like a Nexus One, to trigger it.)