Actually, there are a couple of plugins for self-hosted WordPress blogs that allow commenters to edit their comments for a set time after posting, and I used one for a while, but I’ve been afraid to reinstall it now because I’m pushing against the ceiling of allowable CPU usage at my cheap shared webhost, so have had to cut back on non-essential plugins, plus anything that uses javascript slows down the site. But these plugins might’ve gotten better and faster since the last time I checked — I’ll keep an eye on them.