From what I can tell, there's currently no way to write to StackExchange sites using the API. Are there any plans to support this? This would mean some implementation headaches, such as providing a way to log a user in through the API and then call API methods on behalf of that user (to post/edit questions/answers/comments).
The simplest system for authenticating users through the API would, in my opinion, be something à la bit.ly. If you're not familiar with it, they basically provide each user with their own private, unique, secret API key which they can give to an app and the app will then be able to perform actions on behalf of that user, using that API key. The user can reset his key at any time, if an app goes berserk.