The BBC has a really paranoid article about hackers being able to sniff out your keystrokes via the power circuit to which your computer may be connected. Obviously, this doesn’t apply to laptops, and from the article they’re only testing this out on PS/2 keyboards with really crappy shielding. Apparently due to poor shielding, data can be leaked out into the ground wire which goes to the power supply and ultimately out to the wall outlet. They’ve so far only proved this method works with heavy analysis and only out to about 15 meters away, but with a lot of refinement could happen much further away.
So, if you want security, get a USB keyboard with good quality shielding, put a surge protector between you and the wall, and make sure no one’s snooping around the outside of your house (guard dog, fake cameras, and a fence will help some with this part). Or you could just tighten your tinfoil hat and do your unscrupulous browsing at an internet cafe.