Flushing your DNS cache in Mac OS X 10.5.x Leopard
Simply run
dscacheutil -flushcache
in the Terminal. (/Applications/Utilities/Terminal.app)
If you are running a version of Mac older than Leopard, such as 10.3 Panther, or 10.4 Tiger, the command is a little different:
lookupd -flushcache