User Centric, Inc., a usability consultancy research group, has published a study finding that iPhone's virtual keyboard is nearly twice as slow to enter text compared with devices that have fixed hardware buttons.
The study gave iPhones to 20 participants who were asked to send text messages and emails using the device, while UC searchers monitored their actions. Hilarity ensued. The group found texting on devices with fixed keys to be fast and accurate. On the iPhone - slow and clumsy.
Now, I am in no way a statistician, but there are a number of flaws in this study. For starters, none of the particiants in the study own iPhones...but all owned QWERTY devices, which skews results dramatically. These users were already accustomed to fixed QWERTY keyboards, giving those devices an unfair advantage in the study.
Second, as I have already mentioned numerous times, you become increasing acclimated to iPhone's virtual keyboard with usage. After one month you'll find text input on iPhone to be every bit as credible as a fixed keyboard. Even faster. I make just as many input errors on iPhone as I do my Treo - the difference is iPhone's spelling correction covers my errors, which speeds input dramatically.
As with all studies, take these findings with a grain of salt and a shot of Bourbon.