I'm not sure whether I missed the renewal announcement, or if AMC just never bothered to make one but the site comicbookmovie.com is reporting that AMC has alreadyordered a 13 episode second season of that will begin filming in early 2011:
Filming is set to begin in February, 2011 with no official television debut set for the AMC show's second season, but producer Gale Anne Hurd also confirms that the series will expand to a set 13 episode season. This is to be expected as many of these harder edge shows run around that length.
The first season, which is only six episodes, will premiere on Halloween night (Sunday, October 31 at 10pm) on AMC. The renewal doesn't surprise me after watching the 4+ minute trailer (which is definitely worth a look).
Update 1:00p PT: Eric Goldman of IGN tweets that "AMC tells us WALKING DEAD has not been renewed yet."