According to this Kotaku post, Fallout 3 is to be censored in Japan before its December release. From the piece:
The side-quest The Power of the Atom has been changed. Non-playable-character Mr. Burke has been taken out of this side-quest, removing the option of detonating the nuclear bomb. That's not all, the name of a weapon was changed as it was deemed "inappropriate" for Japan. Smart money says the weapon is mini-nuke launcher "Fat Man" for obvious reasons.
Those wacky Japanese. More through there.