Speaking to The Wall Street Journal, an Apple spokesperson said that 3G-enabled iPads sold in Japan will be compatible only with Softbank, and not from rival providers such as NTT DoCoMo, when used in Japan. However, despite that, those iPads can still be used outside of Japan with any provider who uses compatible micro-SIM cards.
The comment provided to the Journal does not specifically address whether or not the iPad is SIM-locked, but reading between the lines it would seem that the situation in Japan may be similar to the situation in the U.S. with AT&T.
In one of his increasingly common alleged e-mail exchanges, Apple CEO Steve Jobs told one customer last week that the Japanese 3G iPad would accept international SIMs. That would seem to agree with a comment Jobs made during the iPad’s launch in January, when he said all iPad 3G models were unlocked. However, one site reported that the Japanese FAQ for the 3G iPad says that international SIMs won’t work in the iPad—that may mean that you can’t buy a SIM card from another country and use it in Japan.
Despite the clarification offered by Apple, it seems that the situation is still somewhat on the convoluted side. For the full story, we may have to wait until the 3G iPad arrives in Japan later this month.