Hi Adrian,
Actually for ccna security there is one lab section in which you need to configuring AAA on Cisco Routers to Use Cisco Secure ACS. So, from this, I would assume that you would need to know that you can use ACS for AAA and you would need to know commands that would configure a router to use ACS for those functions.
As far as ACS itself, there doesn't appear to be much in the exam topics. However, on a side note, ACS is a pretty cool product, one I have been using for years now.
sample config:-
http://www.cisco.com/en/US/tech/tk59/technologies_tech_note09186a00800946a3.shtml
thanks,
Vinay
Thanks & Regards