The foul-mouthed folks at Ars Technica wondered why their iPhones weren't updating and recognising their, um, custom verbiage. Turns out there's only one way to make sure an iPhone/iPod touch learns the words you use. The iPhone's custom auto-correct dictionary (located at
/private/var/mobile/Library/Keyboard/dynamic-text.dat for all your jailbreaking, SFTP-ing types) doesn't update itself when you're using, say, Notes. Using Mobile Safari's Google search box, however, seems to instantly refresh the file:
... Clearly, not every iPhone application can "learn" words. Notes seems to be brain-dead in that regard. At the same time, using Safari's Google text entry field worked every time, expanding the database and adding new items to my keyboard dictionary.
At the same time, Notes and other type-friendly apps will pick up whatever additions you make with Mobile Safari. Whether that information is worth spending some time creatively searching in Google to train your device is up to you and your time constraints (and finger thickness).