OK, I think I'm losing my mind!
What would cause access-based enumeration (ABE) to become disabled? I have a number of W2k8 R2 servers with DFS Namespaces. The shares associated with each namespace have ABE enabled but, some time later, it becomes disabled. There's no clustering involved, I couldn't find a group policy that affected ABE but it's definitely happening.
I just checked 8 shares on 3 different servers and ABE is disabled for all. I'm going to try enabling it and then rebooting to see if that's what causing the issue. It's maddening!