KDDI has inked a deal with Skype such that the VoIP service will be made available through a Skype Android app on KDDI’s handsets; initially Sharp’s IS03
It appears that KDDI’s partnership with Skype will be similar to Verizon’s in the US, where Skype over WiFi will not be allowed, additional restriction is that Skype to land or mobile lines within Japan are also not possible.
More details at Mobile In Japan.