Data entry with one hand in portrait is definitely tricky. Just making an UI portrait doesn't automatically enable one hand entry. It's a rather limiting area on screen that is easy to press with the same hand that is holding the device. In theory you can try to reach all the corners with the same thumb, but in practice it becomes very cumbersome very quickly.

Data entry, if talking about text input, is also problematic. (And even more problematic with one hand.) You could do a T9 design, but then again, T9 is just far slower than the HW qwerty. The effort of switching to landscape and starting to use the qwerty becomes very small in comparison to the effort of trying to type something with T9. Then again, Apple has a qwerty in portrait. In order for that to work nicely, you really need a predictive engine (as Apple has) on top of it, trying to smooth out and predict your finger presses. (Yes, you can try to live without it, but I'm pretty pessimistic about how long users would be willing to use a portrait qwerty without it.)