Google and Yelp are in advanced acquisition negotiations, we’ve confirmed from multiple sources. And while the deal isn’t done, we’ve heard that it’s very likely to close. The price is supposedly at least $500 million.

Yelp was founded in 2004 as a way to let users leave reviews on local businesses. Comscore puts worldwide traffic at nearly 9 million monthly unique visitors, and it has been growing fast – the company says it’s real numbers are more like 25 million monthly uniques.

Yelp has whispered that 2009 revenues will be around $30 million and are expecting $50 million or so in 2010.