From what I understand, for it to be money laundering, you must be laundering money. But, according to the IRS, bitcoin is not money, therefore if cannot be laundered. The only folks that could be clearly accused of money laundering in the Silk Road case would be currency exchanges if they knowingly traded fiat for bitcoins with folks who were buying or selling illegal drugs.
I'm assume that there is a difference in the legal definitions of "money" and "currency", and IRS regulations apply to taxes and not necessarily to areas of the law. Anyway, your proposed defense was already shot down in the Trendon Shavers ponzi scheme trial.
That's true, the IRS has it's own definitions, regardless of the law. I was under the impression that Trendon Shavers was using that defense to avoid the charge of security fraud. I don't know if he was found guilty of money laundering or not. However, in his case, there was a lot of transactions between USD and bitcoin in order to hide what he was doing.