It took them a while, but DeNA finally released Mobage for iOS on the Japanese App Store on Thursday (iTunes, Japanese only). The free app was made available at the same time as three games, namely Solitaire, Reversi and Ninja Royale.
Smartphone users in Japan were able to access Mobage through the browser since December 2010. DeNA released a dedicated Android app in May, followed by an international version two weeks ago. The international iPhone version should hit the App Store in the following weeks.
Screenshots of Mobage for iPhone (Japan version):