Some more information from the Japenese MIC (Internal affairs and Communications).
Share of Carriers in subscription for Cellular Phones and PHS:
Docomo still lead, while KDDI increase their market share.
As for LTE: KDDI, Docomo and Softbank already have LTE plans, so it seems the future competition in Japan will not be about Cellular Technology and Coverage, but for additional services (and marketing).
Softbank, while pursuing LTE technology and supposedly selected Huawei for trial, also prefers to deploy HSPA+ while waiting for the LTE market to mature and cost to be driven down.