Facebook started the roll out of a major change in functionality for users on Thursday (21 June 2012).
The increased functionality comes in the Pencil menu (top right hand corner of the posts you make) and will allow you to edit comments/images.
Yes, you read that correctly - you're finally going to be able to edit something you've said! Hurrah!
Facebook will be showing an audit trail of changes made so if a friend replies to a then edited post you'll still be able to make sense of the conversation.
The function will take a few days to hit all users, so keep checking by clicking the Pencil icon and eventually you should see the function appear.