http://en.wikipedia.org/wiki/Language_input_keys The button on the Japanese keyboard labelled カタカナ ひらがな ローマ字 seems to do nothing with ibus-anthy. It is supposed to force input into a particular mode without conversion. ibus-0.1.1.20090203-1.fc10.x86_64 ibus-anthy-0.1.1.20090203-1.fc10.x86_64
Kana key (labelled Katakana/Hiragana) should rotate the state of the next pre-edits through Katakana, half-katakana, and back to hiragana (the default): ie the kana subset of the ibus-anthy input modes (not full-width ascii and normal width ascii).
The keysym is Hiragana_Katanaka.
Fixed in ibus-anthy-1_1_0_20090402-1_fc11