First, I would like to say that threading rocks.
My script that was running more than 6000 DNS queries went from 15 minutes and counting (when I hit CTRL-C), to about 30 seconds. I can't wait to try it out against the big list of IP addresses.
Secondly, I would like to give praise to Python for making threads easy to use.
thread.start_new_thread(doLookup, (line, 1))