Forget about what's possible in terms of tracking you on the internet physically, the government can do it though unchecked force.
Under the USA Patriot Act (Slate 4-part series on the Patriot act here), the FBI can compel your Internet Service Provider to provide your passwords and identifying information about you. They do this by writing a National Security Letter (NSL) to your ISP (or library, or credit card company, or....).
There is almost no judicial oversight when the FBI writes an NSL, except for a secret government judicial panel that is just a rubber stamp exercise. When the FBI gets your online info, you are not made aware of this.
The ACLU has filed a case on behalf of an anonymous ISP in New York. The government has put the ACLU and the ISP under a gag order, but some details have gotten out.
I feel the government does need expanded powers to fight terrorism, and that laws must be updated to meet the opportunities new technologies offer terrorists. But we need to hash this one out in the light of day. It needs to be clear where the line is drawn on government snooping, and it needs judicial oversight.
Unchecked government power to surveil it's citizens is a very, very bad thing. See the history of the FBI's past foray's into unchecked surveilance under COININTELPRO if you need any convincing.