Why bother with PGP?
The very nature of email communication makes it perfect for spying and tapping. Imagine all the emails that fly across the Atlantic Ocean. Every single one of them could be tapped without you even noticing it. Of course, thinking that someone will sit and read every single one of them is preposterous - but the "problem" is that nobody has to.

But how do they find out what to tap?
Imagine if the FBI (Or whoever else, for that matter) set up a server that searched all email messages for keywords, say terrorism, bomb, assassination etc. etc. The emails that were caught in this filter would then be inspected by hand, and (obviously) read. So far no problems, unless you consider that you probably don't want people to be reading your emails. Whatever you send in an email is, for technological reasons, highly insecure. It has been said that "emails are as private as anything you send on a postcard". Security is so bad, that anyone who can be bothered to read it can do so as they please.

Of course, most of us, the law-abiding citizens, will not mind people reading emails containing the mentioned words. What, however, if the operators get bored, and add words like "sex" or "secret" or "stock exchange" to the search criteria? They could well be reading your love letters to your girl / boyfriend, your confessions or your business secrets.

The legality
When the USA passed their USA Patriot Act a short while ago (October 25, 2001), after the terrorist attacks on the WTC, in effect they bypassed a law making it hard for the NSA and the FBI to spy on people. So far, bugging a house, or tapping phone conversations involved getting a warrant etc. These warrants were hard to get a hold of, and were often delayed by several days.

With the new legislation, all the NSA or FBI (or even the CIA) would have to do is to claim someone belongs to a terrorist organization. This seems like a fairly wild measure, but there have been reported cases where people have been labeled "Terrorist" just because someone who is also labeled "terrorist" has them in their address book. As you can imagine, this way just about anyone can be labeled terrorist, and just about anyone can have their phones tapped, have cameras fitted in their house, and (obviously) have their email monitored by whoever wants to do this.

Personally, I am not a criminal, and I really don't mind the monitoring of terrorism. But at the same time, I strongly resent the fact that I can't seem to keep my privacy either, because of the mentioned laws and law practices. That's why I urge you to have a look at PGP - Pretty Good Privacy. Free encryption that makes sure that only the recipient can read your emails!

Jetify notes that...

There was a case of a mafiosi in the US who encrypted all the data on his PC using PGP. The FBI obtained a search warrant for his house. While searching (without his knowlege of course) they installed a keylogger on his computer. They gained his passphrase, etc., and therefore had access to his data.

And so it goes, that if you're using PGP on a computer, and they want access, they'll use a keylogger. PGP provides good transport security between readers, but for authorities such as the FBI, there are easier methods of getting the same information, and there are better means of protecting your data from them.

... Which is why you should read Keeping your data from the FBI