Japanese Alphabet mobile application for iPhone / iPod Touch is now on sale at App Store. This helps you to read Japanese phrases when you come to Japan, study Japanese, order something at Japanese restaurants, etc. It's just USD $1.99. Buy this now!
This allows you to enter Japanese in serveral ways. First, Hiaragana software keyboard. Originally iOS doesn't have the feature but the app has.
Second, Katakana software keyboard. Again, you can easily select a Kana character you want to enter by clicking it from the list.
You can also use iOS's built-in Traditional Chinese character hand-writing keyboard. Note that most of Kanjis used in Japan are same to Kanjis used in Taiwan. Differences are intelligently corrected by the app.
At last, you can call Google Translate from the app. Entering English phrases, you will get Japanese phrases translated and see how to read them.