When a comment is edited, a writing utensil icon is appended to the comment. On hover, the alternative text displays:
this comment was edited.
I think, that in some cases, it would be helpful to include a timestamp to this copy.
this comment was edited at 2012-11-21 02:37:43Z or the
@ symbol in place of
Another possibility is to make it similar to edits, where
edited X minutes ago is used.