I would believe such a system is already at work based on US soil, searching for key phrases and the like throughout any and all text based transmissions, and probably extending to a worldwide reach. I've heard a name for this system a few times, but can't recall it offhand.
I agree it does tread very heavily upon privacy. On the other hand, if they're reading mine, they better get some no-doze.
A little OT --
I was subjected to US DOT drug testing years ago. One can't deny that it is un-constitutional, in that the tested are proving themselves innocent prior to be accused of anything, but there was no way around it, and so it was generally accepted. I then went on to being appointed by default, as drug testing administrator for the fleet.
(Obviously no back ground checks are required for the position
)
I gotta tell you, reading the regs on US drug testing could make any law professor stick their head in the gas oven.
