ORIGINAL: carrellh
How long has it been there?
It's hard to tell but it looks like there could be rust or crud in the threads. You might need to put some penetrating oil (such as Liquid Wrench
http://www.gunk.com/lubricants.asp) on it before trying to turn it.
Ummm it was there less than a minute. LOL! The gunk was me trying to cut a slot with a dremel to try and unscrew.