I've added a new mod to the forums that keeps track of your post histories. Sadly it won't have saved the histories of any edits you made before now, but from today on any edits you've made to posts will be saved. To view old versions of your edited posts, simply click on the little "Last Edit: some time" link that appears at the bottom of edited posts.
For you regular users, you should only be able to view old versions of your own posts. Admins, however, are able to view anyone's edit history.