The only scenario I can think of where youd need one hand typing is when your driving, and you shouldnt be doing that anyways.
That's not necessarily true. I've spend the last couple hours reading these threads and trying to find a keyboard that would work with an application I have that requires entering UPC codes. You need two hands- one for the item and one for the phone.
Another reason for a one hand keypad, one for the phone and one for your drink.
Darn it.......... No hands left to drive with, perhaps the knees