I came across this Apple technical note this morning, which describes how to flush the DNS cache on your Mac. We have a hint for 10.5, which is also valid for 10.6, but we're not up to date.
So, as a reminder, to flush the DNS cache in Mac OS X 10.5 and 10.6, run this command in Terminal:
sudo dscacheutil -flushcacheTo do the same in 10.7 and 10.8, run this command:
sudo killall -HUP mDNSResponderAs Apple points out, you might need to do this in the following situation:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120808001623238