[The iPhone 3G is] commanding 24.6% [of the domestic smartphone market], while the [iPhone 3GS] contributes another 21.5%. The 3G was introduced in Japan in summer 2008 (the 3GS followed earlier this year). Sources in Japan say that the iPhone user base in Japan now stands at 3 million, which is an impressive 10% of the global userbase.
They credit carrier SoftBank's aggressive TV and print advertising, and the lack of similar pushes for BlackBerry and Android, as key factors in the iPhone's success. (Android comes in at #10 with 2.3% and the BlackBerry Bold at #11 with 1.2%)
UPDATE: Engadget Mobile points out the smartphone market as a whole is really teeny tiny in Japan,