I asked a similar question (in a roundabout way), in another thread (here: http://developer.myspace.com/Community/forums/t/356.aspx), and this was the answer from one of the MySpace dev team...
Great questions! We will be providing capabilities very soon for applications to make "post" actions via APIs for comments, bulletins, etc. - these will be distinct from the Activities API, and will involve a standard interface for users to confirm the action, but should enable a lot of creativity in constructing new interfaces for MySpace actions.
So, yes, it'll be coming, soon, will not be a part of the Activities API (possibly an extension of MyOpenSpace, but really, could be anything), and because of the confirmation, should limit this as a potential threat from spammers and the like (I assume that this is precisely why they're moving slowly on this one). Ignore the last sentance, that was a response to my exact question.