Yelp , the popular city-guides website fleshed out with user-generated reviews of local business, including restaurants, bars and shops, is about to get a run for its money.

The ever-powerful search engine Google announced on a company-sponsored blog today that it purchased Zagat. This puts a new recognizable–and highly regarded–reviews brand under its belt while getting a leg up in the increasingly competitive local commerce arena.

Founded 32-plus years ago by Nina and Tim Zagat, the beet-colored pocket-handy guides had remained must-reads for those craving to be in-the-local-know and sample only best-of-best fare, from where to shop to where to wine and dine. What it was lacking in recent years, though, was the digital appeal of apps and mobile-device accessibility.

Still, in cities across the world, Zagat approvals carry weight, winning knowing nods when posted in establishments’ windows and luring in would-be passersby with its established name and reviews.

On the Google Blog, Marissa Mayer, vice president of local, maps and location services at Google wrote, “Moving forward, Zagat will be a cornerstone of our local offering—delighting people everywhere with their impressive array of reviews, ratings and insights.”

Mayer went on to write that Zagat’s surveys may be one of the “earliest forms of UGC (user-generated content).”

Such as, could she mean, the user-generated content housed by competitors such as Yelp and, ahem, Foursquare?

Yes indeed, we think so.

Yelp has garnered not only favor but also trust from users with its no-ulterior-motives reviews contributed by everyday samplers (locals or visitors) who experienced spots and, in good faith, simply wished to share their views by posting them to the site. Yelp prides itself on being free for users and “based on the informed opinions of a vibrant and active community of locals in the know.”

Yes, but can it provide instant directions and flashy you-are-here maps accessible via smartphone?

Let us also not forget how fun and game-y Foursquare made publicly checking into locations (from subway stops to cafes and office buildings) by awarding “mayor” titles to those who digitally checked in most often. It also allowed users to post menu recommends (or regrets) and to peruse the reviews of others in the name of the try, review, share mentality now rampant on social networks.

As part of the deal, Google acquires review content on restaurants, hotels and nightclubs to accompany its mobile tools and maps, reports Alexei Oreskovic for Reuters.

Google sees the light. It can no longer compete on geographic and directional data alone. It’s got to bring the user-generated data that mobile-device carriers crave (and incessantly check).

On highly trafficked social-networking sites, word-of-mouth recommendations go far in shaping consumers’ decisions and stop-offs.

The playing field, of course, could have looked very different. In 2009, Google was reportedly considering purchasing Yelp for some $500 million, wrote Oreskovic on Reuters.

Though price details of the Zagat purchase were not disclosed, Michael J. De La Merced, Ron Lieber and Claire Cain Miller reported on The New York Times that “people briefed on the matter said that Google had paid $100 million to $200 million.”

Can Google clobber such new and different ground and pose a real threat to established user-generated review sites? Or is it spreading itself too thin? Think. Review. Share.