On another board I've seen the simple and yet elegant solution of keeping an edit history. Anyone can edit their own posts and any time. And anyone else can click on the edit history to see what the content used to be.

(Oh, how I hate running out of votes before the end of the day -- I'd love to have another ++ on hand for this.)

This technical solution effectively supplements the social sanctions we already have. Sure, you can delete your post or retract an overly broad claim, but everyone will see you doing so.

A good memory is part of building a rich community; people can improve on a bad reputation, but they shouldn't be able to make people forget how they've behaved in the past.