That's a terrible idea ... It sounds awkward and slow, and anyone doing it in public might look like they have mental problems ... I imagine the learning curve would be miserable as well, especially considering you would probably have to memorize the button placement well enough that you could press them without looking at the screen (since you are wavin your phone all over the place the whole time you input text) ...
Imagine trying to send a quick text while walking out in the sun and everytime you enter a character the sun reflects off your shiny screen protector and you flash yourself in the eyes!! ... Not to mention all that wrist movement, yipeee!! carpal tunnel without a keyboard!
...
Next best thing ... I think not ...