I too would like this. I'm discovering the iPhone 4 keyboard isn't really up to par in terms of responsiveness (seems to miss keys and space bar hits frequently when typing quickly), and in terms of customization.
i saw that - i guess you can create words and the replacement word be the exact same word? so if I were to type in "hell" which is typically replaced with "he'll", i'll just use "hell" as the original and replacement word..