Hi Brad.
If your LDAPs are behind load balancer, so this devide takes care about availability and failover. ACS "sees" just one virtual IP. Load balancer, in turn, may use special "keep-alive" packets on speicified port in order to check LDAPs availability and latency.
If you see no failover on you LDAP servers which are behing LB, please, check load balancer configuration first.
P.S. ACS uses no special "keep alive" - you just confugring two LDAP servers and internal logic switches from first to second after timeout.