I must admit, I used to like pocketcm keyboard even before I found this, but I use the skin showcased in this youtube video.
Its not PURE iphone, but its big, useful and works great for fat fingers. I have NO problems texting or emailing fast with it.
Heck I just signed up for an exchange account (found a great 5.00 per month deal) and use it for inputting on cal and notes etc.
Check it out, its from some other cell site
http://www.youtube.com/watch?v=haqujg4nP6I