This could just be some sort of urban legend however NPR’s This American Life is claiming some pork producer may be selling “pork bung”, the rectum of a pig as artificial calamari. Granted, it’s in sausage and most likely other highly processed pork. Some more discussion on CHOW about the product itself.
Update: Slate has some pretty good reporting that suggests this story is bogus and it’s not true. If it’s in hot dogs or other things you eat, that’s a whole other story.