Maybe it's just me, but I'm not really clear about how a situation could arise where the court would demand for the private key to a bitcoin wallet. If the crime was receiving money for some illegal activity, all they would have to do is prove that someone had the private key to the public address. They would have to know something linking you to the public key in the first place to charge you with a crime. Your financial transaction history would be on the block chain, hence, public. I'm not a lawyer, so perhaps I just don't get it.
I could see a potential situation where someone is ordered by the court to pay back taxes or alimony, and it comes up that the defendent has the private key to a 100K BTC wallet. A divorce case might perhaps, but it would be easy for the defendent to move the BTC around and claim that they lost it all on Satoshi dice or something.
I can see a situation where a court of law would demand the private key or keys to a bitcoin wallet. Hint: pirateat40.