That's an interesting feature request, but it's definitely not as simple as a s/8000/80/ search-and-replace.
It has been over a year since I last looked carefully into the CP code, but here is the gist:
When used with SSL, pfSense's CP uses two ports: 8000 (http to facilitate the 302 redirect from http -> https) and 8001 (https for the authentication form where the user inputs his username/password or his voucher code). So if you're going to try to accommodate guests whose system's firewall has a very restrictive egress filtering policy, you'll probably have to use 80 and 443 respectively.
IMHO pfSense's CP needs some developer time to get up to speed with other CP implementations, and to deal with the quirks of modern platforms (e.g. Mac OSX 10.7.x clients will try to contact OCSP URLs of your captive portal's SSL cert).