Version 2.6: WordPress v2.6 gets versioning.
Version/revision saves a copy of a post every time you edit it, allows you to view (not edit) each of those copies (revisions), Shows forward (in time) diffs between any two revisions.
I’m a big fan of versioning. As I discussed in this post, I think it’s a core component of content management, and it’s tough to call a system a true CMS without. This argument managed to persuade Josh Clark, so perhaps Matt Mullenweg read it too?