Interesting story... To answer all posts in one:
-It was edited under photoshop to add the copyright notice and nothing else. It was not oversharpened.
-Conditions : Canon 1Ds MarkII on a tripod, Canon 24-70 f/2.8L, aperture f/8, speed 1/100, focal length 24mm, end of october, around 1PM solar time, very sunny day but not extremely warm (around 12 Celsius), no wind
Just to be sure, I decided to take the original RAW and to run it into another converter (Digital Photo Pro) with contrast and saturation set to normal and with sharpness set to 0 (and 0 is NEVER used by anyone; the least you use is 2, and most people on landscape shots use 5, or even more).
Guess what? The famous "halo" was still there (I was not exactly surprised by the way).
In order to make it more clearly visible, I loaded the DPP image into photoshop and modified the hue (and ONLY hue; no saturation, no contrast, no lightning, no sharpening). With the proper angle of hue, the halo is extremely easy to see especially above the stones of the tower: a clear black/yellow line that separates the stone from the (now orange) sky.
It is less clear along the vertical wall of the tower, and even more difficult to see above the flowers on the left.
So where does it come from? This is not oversharpening (difficult to believe with sharpening set to 0...), not saturation, not contrast and not demosaicing as it doesn't appear everywhere in the same way.
The answer is: it has nothing to do with sharpening; the air is warmer above the stones, and its refraction index is slightly different; it's just a reduced version of the hot road phenomenon.