This is a bit of a vague question, but it seems relevant these days with the information on all those "mass spying" programs getting leaked. I will describe the scenario and I just want to know if it is possible under current laws (specifically, US laws but I would be interested in laws of other countries also), and if so the likelihood of it already having come to pass:
A major corporation with a major website (let's say Yahoo) receives a notice that one of their users is under investigation and they are asked to release all relevant info on said user. Not wanting to cause trouble, they politely comply and nobody except for the government organization and a few people at the company knows what took place. Some time goes by and they are contacted again, this time being told that the user under investigation has taken measures to obscure his/her identity and they are not able to pinpoint them to any one username. They are, however, able to tap the user's line and need the website's private key in order to decrypt that user's traffic. The company hesitantly agrees, the investigation finishes, and the government organization thanks them for their help. However, it's not over. The government organization has in their possession the private key that protects millions of users' data and could pass it around secretly to other entities. If it were given to another government organization that had set up the means to intercept data on a mass scale, they would be able to secretly decrypt all data to and from that website without any billion-dollar supercomputer.
A few incidents like this, and ALL the private keys for the world's top websites could be collected (and it doesn't seem like a very unlikely scenario to me...) What do you think?