3 prong S/PDIF in on mobo?!?

I just received my Gigabyte GA-8PE667 Ultra (which has onboard sound) and am in the process of slapping it into a case. I usually run S/PDIF out from CD-ROM into S/PDIF in on a soundcard (mostly used mobos w/o onboard sound). However, the Gigabyte's onboard S/PDIF in is a three prong connection rather than the normal 2 prong. What am I missing? Is this S/PDIF in for some other device rather than for a CD or DVD ROM drive?