You've seen paper wallets and stuff right? The reason these work and still retain their value is because they have the private key stored on the block chain, if somebody tried this everybody would be able to tell because they'd get some of the notes, check the blockchain and then notice that the private keys etc. are fake.

Counterfeit Bitcoins are an extremely unlikely scenario right now, we've already had exchanges try to fake their trade volume and it was spotted very quickly.