If I were Yelp, I wouldn't be worried - reason: think Google's other failed attempts: buzz, latitude, video (before acquiring Youtube) etc. Google is best at Search, no doubt, but there are certain areas Google engineers just don't get, which is more of the result of its corporate culture and strategic focus. Especially since Google seemed to have failed at creating social-based products many times in the past, whereas review sites like Yelp, is very social.

Of course there are exceptions, like Gmail and Maps. But the mere fact that Google is entering my market wouldn't make me worry. Keep calm and carry on!