The problem is that while it's technically possible to create documents and even write code on a smartphone, it's not practical. The keyboard is too small and therefore it's time-consuming and tedious to do a lot of typing with it.
To solve this problem, what Mozilla Labs has done with their Seabird smartphone concept, is that they theorize that by using Pico Projector technology, a full-sized keyboard can be possible. You can watch the video demonstration here.
Now the question is whether any of the smartphone manufacturers will actually build one.