real soon now
It's on the list for the Real Soon Now architecture.
One reason I didn't do it long ago is the problem of "laundering". I'll be handling that by keeping a revision history. A post that's been revised will have a "revision history" link, which will give access to previous versions of the post.
A minor but important point: Only message bodies will be editable, not subject lines. Subject line editing could create confusion by interfering with your memory of the posts on a thread, particularly those you've read or not read.