Thank you both for your comments so far. I really appreciate your pointers,
Alain, but I must apologize as the example I posted was not very clear. I have
played around with this a bit more, and I discovered that the artifact becomes
more pronounced if I increase the quality settings for radiosity.

High quality radiosity: http://www.freeimagehosting.net/image.php?84dfca2887.png
Low quality radiosity: http://www.freeimagehosting.net/image.php?242bc9ec78.png

You can see the artifact where the green arrow points in the first image. It is
as if the "rgb 0" part of the emissive media is emitting black, but from all I
have read in the documentation that is not how POV-Ray is supposed to work. This
time, at least, I know it is not an optical illusion (you can zoom in and see it
quite clearly on the pixel scale) or a quirk of my display (I tried it on
another display, just to be sure).

What could cause this "shadow" to appear?

Here is the same scene as I originally posted, but with the two radiosity blocks
added and the density of the emissive media modified to highlight the
cylindrical "shadow". Low quality radiosity is "RADtest"; high quality is
"RADfinal". (This scene took a bit over an hour to render on my Intel E8400
using "RADfinal" and the "+a0.0 +am2 +r4" command-line options for
anti-aliasing.)

background { rgb 0 }

#declare RADfinal = off;
#declare RADtest = off;

  global_settings {
     #if(RADfinal)
       radiosity {
        pretrace_start 0.08
        pretrace_end   0.001
        count 1600

        nearest_count 20
        error_bound 0.05
        recursion_limit 20

        low_error_factor 0.5
        gray_threshold 0.0
        minimum_reuse 0.005
        brightness 2//3

        adc_bailout 1/255

        media on
        always_sample off
       }
       ambient_light 0.5
      #end

      #if(RADtest)
       radiosity {
        pretrace_start 0.08
        pretrace_end   0.04
        count 100

        nearest_count 5
        error_bound 1
        recursion_limit 20

        low_error_factor 0.7
        gray_threshold 0.0
        minimum_reuse 0.01
        brightness 2//3

        adc_bailout 0.01

        media on
        always_sample off
       }
       ambient_light 0.5
      #end

      max_trace_level 256
}
//includes
#include "golds.inc"
#include "colors.inc"

//declare textures
#declare gold = texture
{
   T_Gold_1C
}

camera { location <2, 2, -7> look_at <0,0,0> }

light_source {<20, 40, 10>, rgb 1.5 media_interaction off shadowless }

plane { y,-3
        pigment { rgb 0.5 }
        finish { ambient 0 diffuse .3}
}

box { <-1,-1,-1>, <1,1,1>
   texture {gold}
}

#declare med1 = media {
   emission 7
   intervals 1
   samples 10
   method 3
   density {
        cylindrical color_map {
         [0 rgb <0,0,0>]
         [0.8 rgb <1,0,0>]
         [1 rgb 1]
      }
   }
   density {
      function {sin((y+1)*pi*5)*sin((y+1)*pi*5)*0.8+0.2}
      color_map {
         [0 rgb 0]
         [0.5 rgb 1]
         [1 rgb 1]
      }
   }
}

#declare med1cont = cylinder {
         <0,1,0>, <0,-1,0>, 1
         pigment {rgbt 1}
}

merge {
   object { med1cont
      scale <.1,1.1,.1>
      translate <1,0,-1>
   }
   object { med1cont
      scale <.1,1.1,.1>
      translate <0,0,-2>
   }
   interior {
      media { med1
         scale <.1,1.1,.1>
         translate <1,0,-1>
      }
      media { med1
         scale <.1,1.1,.1>
         translate <0,0,-2>
      }
   }
   hollow
   no_shadow
}

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