Over the past few months, we’ve heard regularly about a Japanese company, DeNA, that’s seeking to make waves in the US social and mobile game markets. DeNA has actively pushed the story of its own size and vigor, opening its own venture fund, claiming to be bigger than Zynga and making multiple small-company acquisitions.

Despite all, DeNA’s name still isn’t big in Silicon Valley. That will likely change today, though, as word of a $403 million deal to acquire the mobile social developer Ngmoco spreads. Rumored by Techcrunch a week ago, confirmation of the deal was posted this morning on Ngmoco’s site.

Setting aside the price for a moment, it’s not hard to see why DeNA was drawn to Ngmoco. The latter company is by far the most visibly successful at putting social games and mechanics on smartphones, with games like We Rule, We Farm, Touch Pets and Godfinger.

DeNA itself is in the mobile social space back home, as owner of the highly successful mobile social network Mobage Town. Although Mobage only has about 20 million players, a small number compared to Facebook’s legions of social gamers, Japanese players do monetize at a much higher rate.

The match between the two companies will help each spread already-proven titles and content more easily overseas, reaping extra rewards on the way.

What will have the mobile and social game communities talking about this deal for weeks to come, though, is the price. DeNA is paying $303 million in stock and cash up front for Ngmoco, with another $100 million based on earnouts.

For reference, the Electronic Arts acquisition of Playfish last November was for just a hair less, with about $300 million up front and another $100 million in earnouts. In July, Disney announced its $563.2 million acquisition of Playdom with $200 million in potential earnouts, a price that many in the industry felt to be stratospheric.

But because those two Facebook developers both had very public stats, viewable through services like our own AppData, others in the industry could at least see the metrics that led to the acquisitions. Ngmoco, by contrast, mostly operates on the iPhone, on which Apple extends a corner of its own cloak of secrecy over the developers on its platform.

While it has been clear that Ngmoco has found significant success with its “We” series — We Rule, We Farm and We City — few would have guessed that its success was so significant as to justify an acquisition price equal to what Playdom fetched in November, when it had just ballooned to over 50 million monthly active users and was on track for $75 million or more in yearly revenues.

In fact, it may be the case that Ngmoco hasn’t yet reached that level of growth. Multiple other factors may have weighed into this deal, including Ngmoco’s Plus+ social network, the company’s industry-leading reputation, the rapid growth of the Android and iDevice platforms, the strong exchange rate for Japanese Yen, and DeNA’s own intense ambition to corner the market. (Tomoko Namba, the CEO of US subsidiary DeNA Global, has often expressed urgency in her growth strategy.)

Even taking the above factors into account, this acquisition says that Ngmoco was likely highly successful with its strategy of offering free-to-play titles with microtransactions to mobile gamers, a strategy that only a small handful of companies have pursued. The DeNA deal is a convincing testament to the virtual goods model, if nothing else. We cover the fast growth of the US mobile virtual goods market in our newest Inside Virtual Goods report, check it out for more details.

Ngmoco had taken about $40 million in venture funding since 2008 from iFund, Norwest Venture Partners, Kleiner Perkins, Institutional Venture Partners and others.