Is it true that the power line frequency is kept accurate by atomic clocks?
Here's what wikipedia says:


  Regulation of power system frequency
  for timekeeping accuracy was not
  commonplace until after 1926 and the
  invention of the electric clock 
  driven by a synchronous motor. Network
  operators will regulate the daily
  average frequency so that clocks stay
  within a few seconds of correct time.
  In practice the nominal frequency is
  raised or lowered by a specific
  percentage to maintain
  synchronization. Over the course of a
  day, the average frequency is
  maintained at the nominal value within
  a few hundred parts per million.
  In the synchronous grid of Continental
  Europe, the deviation between network
  phase time and UTC is calculated at
  08:00 each day in a control center in
  Switzerland, and the target frequency
  is then adjusted by up to ±0.01 Hz
  (±0.02%) from 50 Hz as needed, to
  ensure a long-term frequency average
  of exactly 24×3600×50 cycles per day
  is maintained. In North America,
  whenever the error exceeds 10 seconds
  for the east, 3 seconds for Texas, or
  2 seconds for the west, a correction
  of ±0.02 Hz (0.033%) is applied. Time
  error corrections start and end either
  on the hour or on the half
  hour. A real-time
  frequency meter for power generation
  in the United Kingdom is available
  online. Smaller power systems may not
  maintain frequency with the same
  degree of accuracy.