Right now authorities need only "reasonable grounds to believe" that the content of such messages would be useful in their investigation to get a subpoena. However, "probable cause" means they would need to have enough information about the person they're investigating to warrant a prudent and cautious person's belief that evidence will be found in such messages.
According to Ars Technica, only the content of e-mails and online communications will be protected under this new bill; other key pieces of information, such as names, e-mail addresses, IP addresses, and transactional data will not require a warrant. The reason for this, according to former government official Marc J. Zwillinger, is because it's "the type of information prosecutors use to build probable cause that enables them to seek court-ordered access to more sensitive information."
Why you should care
This bill is important, and its ultimate outcome will affect anyone who uses any type of cloud-based services: Web-based e-mail, cloud storage providers, social networks, online chat applications or any Google products. And I assume that's, well, all of you.
Right now, cloud providers have to give up your information to the government if they receive a subpoena. And not just your information, such as your full name, phone number, and address, but the actual contents of any private communication that they have on record. So if you happen to have Gmail messages or Facebook chats that are older than 180 days, they're fair game.
Now, that's not to say that cloud providers (or tech companies, for that matter) will immediately give up your data. Many companies -- Google and Twitter, for example -- will fight for their users' right to privacy and require warrants before they give up information. In piracy lawsuits, many Internet service providers have been known to fight subpoenas by the RIAA.