Viability Of Power-line Intrusions

If I connect my Vista laptop to a power outlet in public, such as an airport terminal, library, or coffee shop, how viable is the threat from someone gaining access to my computer via a power-line network? If the threat is viable today, what is the best method of stopping it?

I know many people will say this is too unlikely and that other security threats are of much more concern to the average computer user, but with the increasing sophistication of ripoff artists either trying to steal your identity or pilfer your financial data I wouldn't assume the threat is not viable and not likely to increase in the future. After all, the federal government and the defense industry utilize TEMPEST, outlining the need for classified "Red Power" systems to protect computers from power-line monitoring and Van Eyck monitoring as well, versus "Black Power" systems that are connected to the public grid. In fact, not too long ago the French were caught conducting industrial espionage by tapping into a local power grid and accessing information via power lines.