While the guys over at iTextSpeed aren't quite the Guiness Book of World Records committee, they probably do have a pretty good grasp of iPhone texting speed. According to this recent youtube post, the fastest iPhone texter comes in at 56 WPM. Considering the fact that most adults don't type faster than 56 WPM on a real keyboard, this is quite impressive.
Even if you don't plan on challenging these serious typing skills, iTextSpeed is probably a great app to have for those of you who say ridiculous things like "I'm all thumbs" and "my fingers are too big". Pick it up, it's only 99 cents.