The answer to the question is that the mobile web is huge in Japan (90 million mobile web subscribers), and it has always been.
Japanese people basically grew up using the web not on PCs but rather on cell phones. Growth began when NTT Docomo, Japan’s biggest cell phone carrier, introduced a proprietary web system (tailor-made for cell phones) to its 55 million subscribers, which was later copied by the other carriers.
That was in February 1999 (!), and since then, the mobile web has been growing steadily. The market is now worth $17 billion ($11 billion is m-commerce, $6 billion is mobile content), and it’s poised to grow even more. For more info, please go to my recent blog post on this topic: http://asiajin.com/blog/2010/07/…
So the answer to the question lies in the history of the Japanese web, and now we have listed ad/marketing companies at the Tokyo Stock Exchange, which generate the most of their revenue through mobile (not PC) ads. Google is also offering search ads on Japanese cell phones.
And what’s also interesting is that the historic growth of the Japanese web can now be observed (in other forms and to different degrees) in other places in Asia, like Indonesia or China where some users may not be able to afford PCs but access the web through their cell phones instead. Needless to say, this has significant implications for the online ad markets in these regions.