To perform a voice search, you tap the microphone button within the app and start speaking; the app transcribes what it thinks you said. Voice recognition performed pretty well in my brief tests this morning, even when I put on a ludicrously strong Australian accent. A nice touch is that if you search for video content (such as a trailer), it will begin playing automatically in a new tab.
The update also resizes the app to work properly on the iPhone 5 and larger iPod Touches. Existing features such as Google Goggles remain in place. See the video above for a fuller demo.