A Boston Globe article about the Patriot Act describes the increased ability of the government to electronically eavesdrop upon people under the Act:
Surveillance tools range from subpoenas for basic subscriber information and caller ID-type tools to searches of e-mail content and real-time wiretapping.The writers of the article also include a reference to the Center for Democracy and Technology (CDT) page on the USA Patriot Act. One of the documents linked to on the CDT page worth taking a peek at is the Department of Justice's manual on Searching and Seizing Computers and Obtaining Electronic Evidence in Criminal Investigations. Note that the date on the manual is January 2001. While the Patriot Act has probably changed some of the particulars, the document is the most extensive compendium I've seen of information from the government on the subject of searches and seizures involving computers.
Under the Patriot Act, agents can subpoena customer payment records to obtain the identity of a user behind an e-mail address. They can also see where people venture in cyberspace. These clickstreams can reveal what people are reading, downloading and even purchasing.
The Patriot Act also allows law enforcers to bypass Internet providers to capture e-mail addresses or even to read e-mail and wiretap conversations in real time. Any federal judge, regardless of jurisdiction, can now approve such warrants.