When moderating comments in a WordPress blog, the usual practice on a standard install is to just replace the text with the moderator’s message. Unfortunately, this results in the offending comment being lost permanently, making it difficult to look at a particular poster’s history. So, it would be very useful to store this from within the admin interface, and whilst we’re at it, to store private notes containing the reason for the decision.
Update 6 July 2013
I spent a good number of hours today hacking on this; it was actually quite quick to get something useful up and running. I heart WP! Thus, I am able to present the wp-versioned-comments plugin: just install it, activate it, and then edit comments as you wish, safe in the knowledge that previous versions will be kept safely in your database.
If you make use of this, please comment below – I should love to see how it gets used. Bug reports and feature requests are welcome.