Hi Golly,
ACS is using DNS lookup to fetch the DC List.
The dns queries it uses are detailed here:
http://technet.microsoft.com/en-us/library/cc759550(WS.10).aspx
(look at SRV Records That Are Registered by Net Logon)
I don't remember exactly which one is used though, but what happen is that once it got the list from the DNS server, it will try the first, and if it is not reachable, it will try the next-one and so on.