Those of you who owned a copy of the PC game "Tie Fighter" may recall a small piece of EU fiction that camed along with it, narrating the adventures of a Tie Fighter pilot who had a friend who voluntiered to be a Stormtrooper. There's a book where Luke Skywalker meets a Stormtrooper who befriends him or something...

So the answer is: according to the EU, no, they are NOT clones. According to GL, who knows (not even GL knows)... But I'm guessing not, I belive the clones were slowly dying and being replaced with voluteers and stuff...