**Update:** As of [tag:api-v2.2], it's also possible to write questions and answers via the API.

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There are at least a few examples in use, of API 2.1 apps, that write comments.  See:

 - [The *Quick Media Solutions* app][1]
 - [The *StackX* Android app][2]
 - [The *SX.el* Emacs app][3]

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][4] that's under development.  It presumably uses an API that's not publicly documented.


  [1]: https://stackapps.com/q/3552/7653
  [2]: https://stackapps.com/q/3901/7653
  [3]: https://stackapps.com/q/3950/11299
  [4]: https://meta.stackoverflow.com/q/190200/148310