The U.S. government -- and likely your own government, for that matter -- is either watching your online activity every minute of the day through automated methods and non-human eavesdropping techniques, or has the ability to dip in as and when it deems necessary -- sometimes with a warrant, sometimes without.
That tin-foil hat really isn't going to help. Take it off, you look silly.
Gen. David Petraeus, the former head of the U.S. Central Intelligence Agency, resigned over the weekend after he was found to have engaged in an extra-marital affair. What caught Petraeus out was, of all things, his usage of Google's online email service, Gmail.
This has not only landed the former CIA chief in hot water but has ignited the debate over how, when, and why governments and law enforcement agencies are able to access ordinary citizens' email accounts, even if they are the head of the most powerful intelligence agency in the world.
If it makes you feel any better, the chances are small that your own or a foreign government will snoop on you. The odds are much greater -- at least for the ordinary person (terrorists, hijackers et al: take note) -- that your email account will be broken into by a stranger exploiting your weak password, or an ex-lover with a grudge (see "Fatal Attraction").
Forget ECHELON, or signals intelligence, or the interception of communications by black boxes installed covertly in data centers. Intelligence agencies and law enforcement bodies can access -- thanks to the shift towards Web-based email services in the cloud -- but it's not as exciting or as Jack Bauer-esque as one may think or hope for.
The easiest way to access almost anybody's email nowadays is still through the courts. (Sorry to burst your bubble, but it's true.)