Even if they are listening in, who the hell is going to sift through the eight hundred million E-mails that have been generated since Tuesday morning that happen to use the keywords “terrorist”, “bomb”, and “Bin Laden”, and find that one that happens to possibly hold a single actual clue?
The more keywords the system is designed to pick up (and key phrases, etc) the greater the number of “intercepted” messages. But undoubtedly, the vast majority of them are false trails: A movie reviewer says a show “bombs”, a mother tells a friend her two-year-old is “a real terrorist” when she takes away his Barney video, and - yeah, I know I’m reaching on this one 
- a store manager says he has “a bin laden with ground coffee.”
The system almost certainly looks for the word “gun”. But how many references do they get to staple guns, glue guns, heat guns, soldering guns, air guns, pellet guns, paintball guns, potato guns or even snow guns?
How many E-mails and Instant Messages do the online gamers exchange that use the words “flamethrower”, “missile launcher”, “machine gun” and “did you see Lara Croft’s…” well, you get the idea.
How about the old military afficionados discussing “towed artillery”, “panzer tanks”, “frontal assault” and “here’s where Adolph Hitler really screwed up…”?
How about the thousands of legitimate online orders from farmers for fertilizer, from collectors making a legal purchase of an antique gun, or from buyers picking up wartime memorabilia?
How about this very post? How about the thousands of kids in paintball bulletin boards referring to their paintball “guns”?
And in any case, does anyone think these guys are stupid enough to write out:
To: OsamaB@afghaniwilderness.org
From: HyJaK@safehouse.com
Re: Completed training today, ahead of schedule since we didn’t bother with takeoffs or landings. Bought several utility knives today, plan on hiding them in carryon luggage. Commander Orange has purchased our tickets using his stolen credit card on Expedia.com. Operatives departing for airport in rental car in two hours.
Etc, etc, etc… ?
Of course not. As another poster said, they’d almost certainly say something closer to:
“Yeah, the picnic will happen on schedule. Bob picked up the hot dog buns and Joe got the paper plates. Figure we’ll leave in two hours.”
Etc. and so forth. Innocuous, simple, and no keyword system in the world would pick up the actual meaning. A trained human probably wouldn’t pick up the meaning…
This is one of those things that people speak of when saying let’s not destroy our own liberties when we strike back at these people. Eavesdropping on our 'mail will do very little or nothing to help prevent future events of this sort, but WILL be an erosion of our right to privacy as US citizens.