This is from the '11 NEC:
406.9 Receptacles in Damp or Wet Locations.
(A) Damp Locations. A receptacle installed outdoors in a
location protected from the weather or in other damp locations shall have an enclosure for the receptacle that is
weatherproof when the receptacle is covered (attachment
plug cap not inserted and receptacle covers closed).
An installation suitable for wet locations shall also be
considered suitable for damp locations.
A receptacle shall be considered to be in a location
protected from the weather where located under roofed
open porches, canopies, marquees, and the like, and will
not be subjected to a beating rain or water runoff. All 15-
and 20-ampere, 125- and 250-volt nonlocking receptacles
shall be a listed weather-resistant type.
Hope this helps.