Neil,
ACS is not tested with NLB but if cluster hosts are attempting to communicate with the ACS using their clustered IP then ACS should reply.
Do you see any hits on acs ? If you sniff the acs interface, what is the source IP address ? Is it clustered ip or clustered host IP ??
Also on acs --->Network configuration add aaa client with host IP and clustered ip . Now see if acs responds to NLB.
Regards,
~JG