This may not seem directly relavent to AIX, however what it does show is that a correctly configured Unix/Linux system is clearly far more secure than Windows and this is highly significant when choosing your corporate infrastructure.
AIX currently lags behind Linux in one major security area and that is the lack of support for iptables. Iptables is a rule-based firewall built directly into the Linux Kernel. AIX does have ipfilt, however this is currently nowhere near as widely supported nor understood.
One other AIX/Linux feature that is definitely not given enough attention is IPSec. IPSec can be used to create a secure private network/tunnel between servers, and because everything is encapsulated at pack level (layer-2), you do not need to worry about complex firewall rulesets. AIX IPSec performance can also be greatly enhanced by offloading the encryption overhead to the actual network hardware/card.