Tempest technology has been around a long, long time - at least 20 years.
Here in the UK it has been used to detect people who were not paying their TV license (a requirement over here to watch TV even if you don't watch any government - i.e. BBC channels!). Just drive around in a van, and pick up the EM signals, and if you find a location that doesn't have a registered payment you find out exactly where it is and send the boys in the next day.
And why do you think that certain government systems have metal shields surrounding them - anything from a PC upwards?
Obviously wireless technology has the same weakness, and if you have anything confidential to send using this technology (e.g. to a printer), I'd be very wary.
