Yep, it was Damon's plan to cure Silas so that he could bring back Bonnie for Elena. You think that Katherine is a goner at first, then it ends with her waking up with the classic moment asking if she's in Hell *g* The grey hair and the tooth falling out begins in the episode directly after that, but oddly a lot of viewers have drawn the conclusion that it was the cure itself which caused that? I guess the widespread belief seems to be that human Katherine started the ageing process simply because she has been alive for so many years as a vampire and it was her time, but she really died (well eventually anyway!) as a direct result of Damon feeding her to Silas and actually losing the cure to him