This looks like a recently introduced bug that manifests on both iOS6 and Safari 6. Has anyone else gotten bitten by this?
What might be a work around? What if we really actually needed that box shadow to have spread=0? Right now we can get the shadow to render by setting the spread value to -1px or 1px
FIDDLE: http://jsfiddle.net/uCseS/2/
It seems like a bug indeed. However, if you use rgba notation for the background-color, the shadow reappears.
http://jsfiddle.net/willemvb/KuhQp/