I primarily work with web servers, and I use lookupd -flushcache all of the time to clear my DNS cache.
This morning was moving some websites around using my newly-upgraded Mac, and I went to flush my DNS cache with lookupd and I got a command not found error. It seems that lookupd has been removed in 10.5 and replaced with dscacheutil.
So now I can flush my DNS cache with dscacheutil -flushcache.
Mac OS X Hints