"Silver bullet" was popularized (if not coined) by Fred Brooks in _The Mythical Man-Month_, and since that book is pretty widely read, the term is well understood. It's definitely sarcastic, but that doesn't mean it's not accurate.

[Depending on your mythology, you can kill a werewolf with a silver sword, and probably other silver objects. Poul Anderson has an interesting treatment of werewolf vulnerabilities in his collection _Operation Chaos_]