One of the key issues going forward will be encryption and password protection.
The New York Times has reported that Federal law enforcement and national security officials want legislation to require all services that enable encrypted communications (like Blackberries) to be able to comply if served with a wiretap order. I wrote about that issue here.
Passwords and encryption, especially when emails or documents are stored in the cloud, is probably the strongest possible evidence that a person has a subjective expectation of privacy in the contents of the documents. And it could provide a nice, easy, dividing line for courts to distinguish between those electronic messages and documents a person wants to keep private, and those the person does not care as much about keeping private.
The follow-up issue will be whether the Fifth Amendment protects a person from disclosing a password or encryption key to the government. Usually, passwords are thought to be like keys and fingerprints – not covered by the Fourth Amendment because they are considered to be physical evidence, not a testimonial admission. However, it is not hard to imagine the government arguing at a trial that because a person knew a password, the person wrote or received a document, or wanted to keep it secret. In this case, the Fifth Amendment may be implicated. Read more details here.