Slashdot has a post about new research out of Penn State that suggests gigabit speed over powerlines. Money quote:
Penn State engineers, Pouyan Amirshahi and Mohsen Kavehrad, estimated in a research paper
released Wednesday that their system could deliver data at close to one
gigabit per second over medium-voltage electrical lines in ideal
conditions, with speeds of hundreds of megabits per second available to
home users
The summary suggests that modification of power lines would be required, and that a repeater would be needed every kilometer.