In the visual editor, I cannot enter Japanse text at all because I cannot see the candidates which are normally shown by the input method. (A typical Japanese input method intercepts key events, shows candidate converted strings with kanji, and lets a user choose from them.)
My environment is Ubuntu 11.11 and IBus 220.127.116.1110818 with Anthy. I guess the same situation happens with other Japanese input methods, and other input methods which try to show candidate lists at large, including Chinese ones.
Most probably this is related to bugzilla:33056, but I'm filing separately, just in case.