It appears that the x and y offsets to MagickShadowImage are ignored. Regardless of the settings, the shadow is not offset any more than the "default". The "default" seems to change depending on the sigma value. Higher sigma values give a larger offset.
This is with 6.3.6-1 - also tested on 6.2.4-1 as I had that install available as well. Used on both CentOS4 and FC3.
Thanks!