@Yuugi's:
Wait, where does Toriyama say this?
It sounded to me like Wakamoto was saying 'Biiiiiiiiiii!!' actually…
Toriyama said that during the developing of the Budokai Tenkaichi game series. And so Wakamoto voiced Imperfect Cell with a bit higher pitch in his voice in Sparking Meteor AKA tenkaichi 3 in US/EU (his semi-perfect and perfect cell voice remained the same in that game). It seems this has also transferred over to Kai.
And only just noticed: The opening has been updated with images of Cell.