I think this is a bad idea, and I hope punbb will never implement this feature.
1. Can cause severe slow down ,and if your DNS forwarder is down, you may screw up your forum,and cause a self D.o.S
2. Gives you no real security, DNS names are a piece of cake to change/spoof
3. It's not he job of a Forum, you have plenty of tools to do this ( htaccess files, httpd.conf)
The Internet mail is frecuently screwed up by DNS problems, do not broke forums too ¡¡