While I love my iPhone with a passion, I have to admit that the Apple team could have put a little more thought into what features would be needed in Asia. One of the major shortcomings of the iPhone for users in Japan (and I would guess other Asian countries as well) is that your input dictionary doesn’t learn.
iPhone-chan (or as I call my iPhone: ai-chan) tries to detect words contextually (I think…), but any non-standard character usages you manage to get into the system are quickly forgotten.
Fortunately, there’s a workaround. And don’t worry, you don’t need to jailbreak your phone to get it to work.