Confirming yesterday's story, AMC has issued a statement officially announcing that Glen Mazzara will be taking over the role of showrunner from Frank Darabont on The Walking Dead:

"The Walking Dead’s Glen Mazzara, writer and executive producer, is expanding his responsibilities to assume the role of showrunner. AMC is grateful to executive producer, writer and pilot director Frank Darabont whose contributions to the success of The Walking Dead are innumerable. We continue to discuss his ongoing role with the series.

Robert Kirkman, creator of The Walking Dead graphic novels, executive producer and writer on the series, will remain in this role and will continue to steward the brand. Gale Anne Hurd will remain in her role as Executive Producer. The production continues on schedule for an October 16th premiere."

Oddly enough, there is still no mention of the reason for Frank Darabont's departure, but it appears they are still trying trying to have him continue to work on the series in some capacity.

Source: Deadline