Don't forget about the Patriot Act...

Under it's provisions many government agencies for whatever reason can go to your ISP or any server and look at your records. But that's not all, it is a crime for the ISP or server to tell you that someone is looking at your activity...

The Electronic Frountier Foundation says...
http://www.eff.org/Privacy/Surveilla...orism/PATRIOT/
The law dramatically expands the ability of states and the Federal Government to conduct surveillance of American citizens. The Government can monitor an individual's web surfing records, use roving wiretaps to monitor phone calls made by individuals "proximate" to the primary person being tapped, access Internet Service Provider records, and monitor the private records of people involved in legitimate protests.
PATRIOT is not limited to terrorism. The Government can add samples to DNA databases for individuals convicted of "any crime of violence." Government spying on suspected computer trespassers (not just terrorist suspects) requires no court order. Wiretaps are now allowed for any suspected violation of the Computer Fraud and Abuse Act, offering possibilities for Government spying on any computer user.

Here's a great example of Patriot Act misuse I say at Slashdot this week...
http://www.caedefensefund.org/

If you post at Yahoo they say...
NOTE: Do not assume you are anonymous when posting to Yahoo! Message Boards. Although your IP address is not displayed on your post, Yahoo! does record your yahoo_id (alsoaswell) and IP address (2xx.25x.2x8.2x3) to help combat abuse. Law enforcement agencies may require Yahoo to disclose information regarding message board postings. Before you post, please read our Privacy Policy.

So be careful what you do or say unless you are protected...