Isn't it weird how "jap" is offensive but "brit" is not?
The word "Jap" has offensive connotations dating back to World War II, and the particularly cruel and intense fighting and poor treatment of POWs by the then Japanese military.
Post World War II after "Little Boy" (the nuclear weapons dropped on Hiroshima and Nagasaki, crippling the entire country) the Japanese all but closed their doors to the rest of the world, opening them 40 years later and announcing themselves to be a peaceful nation that would never forget the horrors of the war performed by either side.
Many Australian "diggers" fought the "japs" in Papua New Guinea and surrounding areas, and you'll still find older members of the community using these terms and having some pretty severe issues with Asian people in general. Much like the syndrome faced by ex-Vietnam battlers under similar circumstances.
Anyways... you get the picture. I'm not usually one for political correctness; I think it's all a load of tosh generally. But you should at least try to understand where these issues stem from, and in certain circumstances (particularly relating to the horrors of war), showing some sensitivity is a good thing.
[EDIT] I should add, I've spent some time in Japan, and I've found it to be the most fantastic place. The people are the friendliest I have EVER witnessed (and I've done my fair share of travel), and will go out of their way to help a total stranger in need, reguardless of language barrier or cultural differences. These "bizzare japanese stuff" websites paint these marvellous people as some sort of mass deviant culture, and nothing could be further from the truth.
It is a beautiful country with fantastic people, and anyone planning holidays in Asia should definitely consider Japan as a part of their travels. Not doing so would be a great loss to any travellor.