Interestingly, many of the 'alternative' layouts don't have the same advantages on the phone.[1] For example, any of the layouts that emphasize home row locality, such as the * aks, don't really help when you're typing with two thumbs. The Sholes "slow you down just enough to avoid a physical constraint" method turns out to be useful when you have two digits flopping around with the potential of being in each other's way.
Also, you can change the default keyboard at the application level[2], but this leads to annoying "edit here, copy, paste there" workflow solutions. You can also change it at the system level if you have a phone that can be jailbroken.
A particularly awesome example of alternative keyboard implementations on iOS is Hipjot[3], which affords two-finger swype-style typing. The developer demonstrates 120wpm touch-typing on his phone[4] and advertises 70-90wpm on the Hipjot app store page.
