This story made it onto Cute Overload, and several people gave more details in their comments (so none of this information is original to me, I'm just repeating what they said over there). Apparently the cheetahs didn't actually let the impala go, Daily Mail made up the ending. Here's what actually happened after the impala ran away:
http://www.biosphoto.com/consulter/D...rBySpecifique=
http://www.biosphoto.com/consulter/D...rBySpecifique=
(search
http://www.biosphoto.com/ for cheetah and impala for the full series)