It depends on what you're trying to do. Most phones I've used can be operated with one hand, blindly, to do common tasks (raise/lower vol. silence call).
The biggest problem, though, is that modern portable devices /need/ the expressiveness of a keyboard. Which is why devices like the Blackberry, the sidekick, etc. took off.
The touchscreen changes some of that, but, from what I hear, it doesn't quite work as well.
