I had wondered about phone keyboards. Part of the reason I switched back was that I couldn't change the layout on my phone and got tired of switching back and forth. (I know there are probably some options now, but there weren't on iPhone 4, Kindle touch, or feature phones with physical keyboards.)

I can imagine you'd need to use both thumbs extensively or you'd be jumping back and forth. I definitely found that when I was laying on my side on the couch, propped up with one arm, qwerty was easier to use with one hand. Not that that's an ergonomic or sustainable way to type, but I was a lazy college student and I didn't care.