According to Norman Reedus' twitter account, he will be playing an undisclosed role in AMC's upcoming comic adaptation of The Walking Dead. Since all other main characters have been cast for the most part, Reedus might be playing a new character created strictly for the TV show.
Reedus is probably most well known for his work in the cult classic
The Boondock Saints and it's sequel that came out late last year. He has a little experience in comic book movies though as he had a supporting role in Guillermo Del Toro's
Blade II.