Should I cover my air conditioner for winter?
I have never covered mine and this article also seems to recommend not covering it:


  Your central air conditioning unit
  consists of a compressor and
  condensing unit placed outdoors in a
  metal housing. These units, built to
  resist the weather, generally do not
  need a cover. In fact, covers can
  cause problems because they trap
  moisture and create an inviting winter
  home for small animals.
  
  Professionals who service the units
  tell me that most of the damage they
  see in spring was caused by rodents
  living in the units and chewing on
  wiring.
  
  If your air conditioner is subject to
  falling ice or other debris, you could
  cover its top with a piece of plywood,
  plastic or metal held in place by a
  weight.
  Reference: http://misterfix-it.com/blog/?page_id=36#cac


They make a great point that a cover would provide a perfect winter home for animals which would cause damage to your air conditioner.