The keyboard autocomplete is not really so secret: i managed to somehow activate it on the Japanese keyboard without changing any preference files.
It's not even new--been around since iOS 4.1 at least (and actually worked better on earlier iOS versions)
They have been enabling this feature for kana-kanji conversion in Japanese keyboard since when iPad was first introduced (iOS 3.2) and enabled it for iPhone in iOS 5.
And that's why it's very dangerous for iPhone developers to assume that the keyboard height is always 216 points; suddenly your East Asian users could start having problems with your app.