Lots of tutorials out there, but I figured I’d slap in here for the sake of it.
On Leopard you need to use dscacheutil, which will clear it up for you;
dscacheutil -flushcach
On Tiger (and some upgraded versions of Tiger to Leopard… weird)
lookupd -flushcache
This will help clear up your situation. Now remember, that just because your Mac has cleared the cache, doesn’t mean your browser has. You will still have to clear it in this way.