I have some cross platform DNS client code that I use for doing end to end SMTP and on windows I can find the current DNS server ip addresses by looking in the registry. On the Mac I can probably use the SystemConfiguration framework as mentioned in the first answer, however the exact method of doing so is not immediately obvious.
For instance SCDynamicStoreCopyDHCPInfo returns some of the dynamic DHCP related data but not the DNS server addresses.
They are also available from /etc/resolv.conf