The iPhone's virtual keyboard has some interesting methods for improving typing accuracy. As previously reported, it can dynamically change the "hot zone" for for a given key to be larger if it expects you will type said key next. If, for example, you type "fiel," the iPhone will automatically increase the size of the "d" key's hot zone (since no common words other than "field" start with "fiel"). This way, even if you don't precisely hit the "d" key, the iPhone will be able to better know (or guess) what you meant.
A learning mechanism also exists in the iPhone keyboard, and while we're still figuring it out ourselves, here are some observations about how to control the built-in dictionary:
Adding words to the iPhone's dictionary The iPhone will automatically suggest corrections, but what if it's wrong? You can accept corrections by simply pressing the space bar after typing your word, or you can reject corrections by typing the word as you want it, then tapping the word you just typed. The second time (for some reason, it has taken our iPhones three times on some occasions) you do this for the same word, the iPhone will add the word to its dictionary, and suggest it as a correction the next time you begin typing it.
Clearing custom additions to the dictionary What if you've accidentally added words to the iPhone's dictionary, or want to clear the database of custom-added words? Simply go to the Settings application and tap General, then tap Reset and tap Reset Keyboard Dictionary.