Forwarding/authorative DNS (appliances?) with an extra bit of security

I know I just asked about DHCP appliances but how about DNS? We're currently just running mixed environment (~30k users) with Windows DNS/BIND as internal DNS and BIND as external. We have Fortigates, F5's etc. that all could provide some sort of "Advanced DNS" stuff at least according to their datasheets. There's just so many different DNS products with different levels of security, and also lot's of services you could buy...

I guess clustering DNS services is quite easy (HA pairs and anycast addresses...) but how about those security features, what do you use or would like to use?

Any other ideas or thoughts?

Thanks!