If they make it a crime then people who are accepting it for their business will stop doing that and then it will become useless to have it actually, what's the point of having a currency which you can't spend anywhere or buy/sell?
Folks on here often enjoy making comparisons with drugs. If it's illegal they'll just want it more. In Bitcoin's case, er, no. If it's illegal then it becomes utterly useless to almost everyone. Drugs carry on being fun no matter what their legal status is.
One important question in this scenario: What legal basis could be used to prohibit the possession of/transacting with bitcoins? For instance, under Executive Order 6102, possession of monetary gold was criminalized on the basis that hoarding of gold was stagnating the economy. Naturally, this argument falls flat on its face in regards to bitcoin, as bitcoin is virtually irrelevant to the global economy.
So what could it be, then? Association with drugs, illegal goods? Of course, all forms of money can be associated with such. I'd really be curious to see what legal position would be used to justify this.