are you kidding? After being here for four years I always run into Japanese who speak much better English than I speak Japanese. You seriously don't need to know a lick of Japanese to live here your entire life. It helps but is not needed simply from having so much English support all over the place.
I really wish people would stop posting about things they don't know about. I certainly won't try to explain Pakistani and Indian caste social systems, so please don't talk about things you don't know about just to look smart. You make yourself look like an ass when someone comes around who knows what is correct.
I can completely respect that you want to share the knowledge you have gained through all of the A/V you have watched, all of the cartoons and Japanese comic books you have seen, but unless you have first hand experience, please don't comment. It annoys people and leads others the wrong way.
So to re-cap:
1) Going to Japan? Good for you! Consider learning some Japanese
2) Do you need Japanese to visit Japan? Yes and no.
__2a) Will you visit and stay in a large city? Yes? Then don't worry about learning Japanese
__2b) Will you visit and remain in a small town? Then consider buying an electronic translator/dictionary and carry a pocket dictionary with you. Learning Japanese for a short visit is a moot point unless it's your hobby.
__2c) Going to a major city, then traveling around via Shinkansen? Yes, consider learning some general phrases in Japanese.
3) Will you need to use your Japanese while in Japan? Most likely not, plus the vast majority of places you will visit (sight-seeing I presume) will have more than enough English support for you, plus most Japanese speak English to a certain degree.
4) Why am I coming off as a bit of a jerk? Because I know better than most of the online guru's that have been spouting off false information in this topic area ("Japanese Discussion"). Simply put, I'm smarter.
Any more questions: Feel free to ask them here or in private. (Or in the picture thread)
Aqua