We've just received this joint statement from AMC and The Walking Dead showrunner Glen Mazzara:

Today, AMC announces the season 4 pick-up of The Walking Dead. 

AMC also jointly announces with Glen Mazzara today that for future seasons, the two parties have mutually decided to part ways.  Glen guided the series creatively for seasons 2 and 3.  AMC is grateful for his hard work.  We are both proud of our shared success. 

Both parties acknowledge that there is a difference of opinion about where the show should go moving forward, and conclude that it is best to part ways.  This decision is amicable and Glen will remain on for post-production on season 3B as showrunner and executive producer.

Mazzara is credited with writing six episodes of The Walking Dead and has served as the series showrunner since the departure of Frank Darabont.

The Walking Dead is currently on hiatus, but the second half of Season 3 premieres on Sunday, February 10th.