There are at least a few examples in use of apps that write comments.  See:

 - [The *Quick Media Solutions* app][1]
 - [The *StackX* Android app][2]

<br>
Re:

> Am I correct in thinking that only comments can be written and not questions?

Yes, that's correct.  Except for [the super-secret, official, Android app][3] that's under development.  It presumably uses an API that's not publicly documented.


  [1]: http://stackapps.com/q/3552/7653
  [2]: http://stackapps.com/q/3901/7653
  [3]: http://meta.stackoverflow.com/q/190200/148310