Howtoforge published a guide about running ISPConfig on port 80 using Apache's reverse proxy feature

This article shows how you can configure a Debian Etch system that has the webhosting control panel ISPConfig installed so that ISPConfig can be accessed on port 80. By default ISPConfig uses port 81 which is a non-standard port and is blocked by some firewalls and ISPs. By using Apache's mod_proxy module, we can avoid this problem. It lets us create a reverse proxy that can fetch the pages from ISPConfig on port 81.

Running ISPConfig On Port 80 Using Apache's Reverse Proxy Feature (Debian Etch)