Mac OS X, now just OS X, seems to change the way the DNS cache is flushed every couple of releases.
Below are the various commands to flush the DNS cache from Mac OS X 10.4 Tiger to OS X 10.9 Mavericks.
OS X 10.9 Mavericks
Apparently in Mavericks this is a two step process.
Flush the DNS cache:
dscacheutil -flushcache
Then reload DNS:
sudo killall -HUP mDNSResponder
Mac OS X 10.7 Lion and OS X 10.8 Mountain Lion
sudo killall -HUP mDNSResponder
Mac OS X 10.5 Leopard and 10.6 Snow Leopard
sudo dscacheutil -flushcache
Mac OS X 10.4 Tiger
sudo lookupd -flushcache
