Here's an excellent thread that has pictures of this phenomenon taking place with a spider...

http://ball-pythons.net/forums/showt...er-turn...gray

It's rare, but it does happen. So far as I know, no one knows why it happens. The amount of time it takes to return to normal can be anywhere from a few sheds to a few years. hgrub's spider had only regained about half its color after a year and a half....haven't seen an updated picture since then, though.

I've never heard of this condition being harmful to the snake in any way, so don't fret about it. Just enjoy the mystery. (and post pics!!)