Would it surprise you to know that the iPhone possibly isn't the best handset if you spend most waking hours tapping out messages?  No, I didn't think so, but fresh from the "stating the obvious" research file comes the third and final part of User Centric's examination of just how finger-friendly an on-screen keyboard can be compared to traditional thumbboards and T9 predictive text.  In their last conclusion they decided that new users of the cellphone would experience trouble transferring from hardware keys; now they're saying that the Apple handset "may not be suitable for heavy text use".

While speeds eventually reached some sort of parity with that managed on QWERTY and T9 cellphones, the differentiating factor was apparently accuracy:

"The iPhone was clearly associated with higher text entry error rates than a hard-key QWERTY phone. The finding that iPhone owners made more texting errors on iPhones than their hard-key QWERTY counterparts (on their own QWERTY phones) suggests that the iPhone may have a higher fundamental error rate. Specifically, the high rate of false alarms for iPhone keys adjacent to high frequency letters is troubling. The iPhone’s predictive and corrective text features do alleviate some of the errors users make while texting, but it does not catch them all" User Centric report

Bear in mind, though, that User Centric only used data from around 34 individuals, made up of iPhone owners and non-owners.  Their overall conclusion still stands though: if you're tempted by the handset, make sure to spend some time trying one out in your nearest store, first.