I had a look again at the story today, and surprise the word "first" has been deleted from the title of the page. Here is the original title:
Ha'aretz did not post a correction (or at least I haven't found it).
The Guardian continues to publish the story, despite it being debunked.
I have been told that on the printed edition of Ha'aretz (24.08.12) there was a short correction stating:
"בניגוד לפרסום בעמוד הראשון (הארץ, 19.8) אין זו הפעם הראשונה שמשרד החוץ של ארה"ב מגדיר אלימות מתנחלים "טרור".
"Contrary to that stated in the front page (Ha'aretz 19.08) this was not the first time the US Department of State classified Settler violence as terror".