Some testing by CNET and their Xtreme typing challenge had typing in some extreme conditions pitting an iPhone against an EEEEEEEEPC.
Their completely unscientific test came to the same result as I have... while the iPhone isn't tactile, the smarts in the software (auto-correct, guessing what the next letter will be and increasing the hit box for it, etc) makes it far easy to just stab along quickly, and for the most part will figure it all out properly. Course, if you're hacking out an email on the train, the jostling around you're going to get isn't the same as what this guy is experiencing :)