With the introduction of Leopard, lookupd disappeared. A new utility was put in it's place, with much of the same functionality. In particular, it was changed to a new command dsmemberutil.
is now the command used to flush DNS entries, or any other entry held by the Directory Services daemon.
Even better, you can also check on anything that directory services knows about with the following command
dscacheutil -cachedump -entries
You should notice both internet cache entries, as well as local directory service entries, or any other entry that is in the directory services cache. It is a great troubleshooting tool.