This is a boon for those who have to write a lot of Japanese, as there is a lot of switching between the two syllabaries (Hiragana and Katakana). Aside from putting the Input Menu in the Menu Bar
(System Preferences » International » Input Menu Tab » Turn on Kotoeri, and make sure Show Input Menu in Menu Bar is checked), and using the Command-Space Bar and the Command-Option-Space Bar shortcuts
(System Preferences » Keyboard & Mouse » Keyboard Shortcuts; deselect Spotlight; select Input Menu), you can switch between Hiragana and Katakana on the fly by holding down Shift while typing.
As an example, typing in わたしは レイ です。requires Hiragana, Katakana, and Hiragana again. The quickest way to write this sentence starting from the US English Keyboard would be as follows (assuming you enabled the Command-Space Bar switching method:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080305081011567