From the Colts Website

Today, we have placed Corey Simon on the NFL Non-Football Illness/Injury list. In deference to Corey's privacy and in compliance with federal medical privacy laws, we may not discuss the particulars of Corey's condition other than to say it is an illness not an injury and is unrelated to the knee surgery he had in August.

Sucks for Corey. It's been sad to watch such a promising career go so far downhill. I was such a huge fan of him when he was in Philly, then he just decided to stop caring and has never regained the form he had early in his career