A while back, we reported on a study that (not only had all kinds of holes in it, but) claimed the iPhone's keyboard was two times slower than other phones. But as many commenters said, the keyboard just requires practice.
And there's no better way to get it than to jump into a typing tutor. We've seen one before for the iPhone, but reader Travis (thanks!) sent us a tip about TypingWeb.com's free iPhone tutor (just go to their site on your iPhone, and enter some information to create an account), and I was impressed by how smoothly it worked. During the Basic test, I moved pretty quickly-- a nice 36 words per minute. But on the Advanced test, they throw all kinds of things at you (including intentionally misspelled words, so you have to dodge Apple's corrections), and I dropped down to about 16 wpm.
Definitely worth a try. A lot of iPhone typing seems to be situation-- moving around in a car or bus, for instance, makes things a lot harder. But at least this will give you a ballpark figure of your typing prowess.