Erica Sadun has written an article over at Ars all about teaching your touchable pocket pal some nasty habits. If your language is generally more "explicit," or you just have a need for the iPhone (or iPod Touch) to learn someone's last name, then take heart, ye weary iPhone typists!
We previously covered TJ Luoma's tip that involved adding a new contact whose name was a word that does not appear in the system dictionary; TJ has since updated the tip to note that you can include the target words outside the contact name and the approach still works. Erica has suggested a slightly different approach that does not require you to salt the address book with contact fluff. This is especially useful if you have young children or mothers who might happen to peruse your contacts.
The general assumption is that after correcting the auto-correct, the system will eventually add whatever it is you're typing to the dictionary. As it turns out, not all apps are created equal in terms of updating the dictionary with new words. I won't spoil the surprise, but I will tell you that the Notes app is not where you should conduct your keyboard-training session.
This excellent tip can also be used for teaching your tactile friend some less abrasive words that are not in the dictionary. Erica's article walks through the procedure for updating the built-in dictionary on an iPhone or iPod Touch. If you happen to have jailbroken your device and installed SSH, she shows you how to directly access the dictionary. Hit the read link for the full scoop on how to update your dictionary with your off-color language or otherwise unique verbiage.