Currently on MonoX Wall one can delete their older posts but can't edit them. It used to be like that with Facebook too, but some time ago they changed that allowing you to edit older posts.
However, Facebook has gone one step further and shows an "Edited" link under such edited posts that when clicked shows the Edit History! This is probably to prevent people editing older discussions to make other people replies to them appear bad (by changing the questions asked) - in such cases the edit history does help. But the edit history doesn't help much if you said something you want to correct (e.g. leaked some info that shouldn't be there) and want to change it without the old info remaining in the system, neigher want to delete the post and break the logical sequence of the conversation thread.
What I suggest is to have a web.config option to allow users to edit their posts on Walls and not have an edit history feature (not sure how many users have discovered it in Facebook at all anyway).