Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Unless the case includes a magnetron, you should be safe. 🙂

Seriously, though, NFC still works through radio waves. Cases won't affect it. The whole "tapping the device to another device" thing is a gimmick. They don't need contact, just close proximity.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.