I don't recall the exact details, but there were several freepers back in the clinton days who got paid visits by the Secret Service for things they posted here.
It's not as bad under a decent president like Bush as it was under the slimebag, but there are a few general rules to observe:
Never joke about harming the president.
Never joke about bombs in an airport.
Never tell the IRS that the income tax is illegal and you refuse to pay it.
These activities touch the government on its sore points.