I have said this many times, and just in case you missed it, I really HATE the iphone keyboard.  Yea, I used to be a Treo user and really liked having a real keyboard. I normally tell people who do lots of texting or phone based emails, to get a blackberry instead of an iphone for just this reason. That being said, the iPhone, for a virtual keyboard has a few interesting functions to try to make typing easier; autocorrect, word prediction that will actually changed the size of your keys as you type (which can be a problem at times).  So based on these two features, having a good lexicon for your autocorrect can make a huge difference.

The normal way to deal with this is to simply type the same thing over and over again, and it will learn your new word, but it does not always work for me.. I started doing some research on came across this article by, who else, By Erica Sadun …What did I uncover here today? Well, clearly, not every iPhone application can “learn” words. Notes seems to be brain-dead in that regard…. Reflects what I have experienced.  The fix? update the file yourself

..That database is stored in the “mobile” user directory, in /private/var/mobile/Library/Keyboard. The file is called dynamic-text.dat. It consists of a (mostly) plain text list of words that the iPhone has marked for special attention. Once a word appears in that list, the autocorrect feature updates itself and accepts those items as proper spellings..

You need to SSH to your phone and thus need jailbreak. If you are NOT jailbroken, it seems typing in safari is your best bet to make sure the autocorrect list gets updated.

BTW, I heard of an app called iwordTrainer, but I cannot seem to find it. Anyone used it?