The NGA is in between big photo shows and I don't think there are any major photo exhibitions elsewhere, but my current favorites are the Nam June Paik show at the Smithsonian American Art Museum, the Outwin Boochever Portrait Competition at the Portrait Gallery (double score--they share the same building). If you're there, you can also see Pictures in the Parlor, an exhibit of hand-colored tintypes and photos. The Smithsonian Castle has a nice civil war photography exhibit, including camera and luscious Harrison Globe lens.