To enable international keyboards, select Keyboards from the general settings. Tap International Keyboards and then Add New Keyboard. Once you've made your selection, you'll notice a globe icon next to your keyboard's spacebar. In iPhone OS 3.1.3, you could tap that icon to switch from one keyboard to the next. It was kind of a hassle for those with several keyboards enabled.
The iPad introduced a popup menu to the globe icon with OS 3.2 (at right), which the iPhone inherited with iOS 4.0. Now you can simply press and hold to get a list of all enabled keyboards and make your selection.
The other tip is about kaomoji, those pesky little "face letters" that the kids are so into. iOS 4 has made them even easier to use. With a Japanese keyboard enabled, hit the "123" button and you'll see an emoticons key in the corner. Tap it to bring up a list of options. There's a screenshot of this after the break. Enjoy!