This is a bit of a kludge, but will allow your users to choose to remove the Posted via Mobile Message - I will look into a plugin to force the addition of the message also, but in the mean time:
Edit your "newreply" and "newthread" templates for this style and replace:
Code:
<textarea name="message" rows="10" cols="15" style="width:100%; height:100px"></textarea>
with:
Code:
<textarea name="message" rows="10" cols="15" style="width:100%; height:100px">
Posted via Mobile Device</textarea>