These few photos are an unscientific comparison of the Canon EOS 20D 8.2MP Digital SLR and the Canon EOS 1Ds Mark II 16.7MP Digital SLR. The lenses are different, the models are different, and the light is different.
What is the same? The photographer, the flash (Canon Speedlite 580EX), and the post-processing.

All photos were shot in 'raw format', converted to 16-bit tifs using Bibble Pro V.4.1, and edited in Adobe Photoshop CS.  I used a stair-step reduction (10% increments - bicubic) to resize both the 8MP and 16MP images
to either 400x600 or 533x800 pixels before processing. After brightness/contrast, levels, etc. were adjusted, unsharpen mask (267, 0.3, 1) was applied to the Lightness channel in the Lab Color Mode to avoid color
haloing. I used different distances of the models from the camera to compensate for the 1.6x magnification factor of the Canon 20D sensor.

My conclusion... At the same full crop size, the Canon 1Ds Mark II produces a slightly sharper and cleaner image. The difference between the two cameras would probably be even greater if I had used the same prime
lens on the 1Ds Mark II. However, whether this difference is detectable in resized images is debatable. Both cameras produce stunning images.

Thanks to Canon Singapore Pte Ltd. for the loan of Canon 1Ds Mark II. Also, thanks to Eric Hyman of Bibble Labs for taking time to communicate with a photographer living on a red dot on the other side of the world.
And of course, thanks to the lovely ladies... ever patient.