Unfortunately, Cain's mark wasn't forgotten by the writers(though I'd rather it had been.) In later issues, Cain went on to rid himself of the mark by passing it to The Question(Renee):
http://i.imgur.com/mqEq5du.jpg
http://i.imgur.com/FwJi3oE.jpg
http://i.imgur.com/lhOp9gk.jpg
http://i.imgur.com/zQ2IIyY.jpg
http://i.imgur.com/78n90Bn.jpg
http://i.imgur.com/sggS9Jb.jpg
IOW, he passed the judgement of God himself to an innocent. facepalm
*The Cain origin wasn't adopted by the DCnU, however. His story is now more akin to the 'classic' Vandal Savage we all know and love.